Platine War

The Platine War (Spanish: Guerra Platina, Portuguese: Guerra do Prata, Guerra contra Oribe e Rosas; 18 August 1851 – 3 February 1852) was fought between the Argentine Confederation and an alliance consisting of the Empire of Brazil, Uruguay, and the Argentine provinces of Entre Ríos and Corrientes, with the participation of the Republic of Paraguay as Brazil's co-belligerent and ally. The war was part of a decades-long dispute between Argentina and Brazil for influence over Uruguay and Paraguay, and hegemony over the Platine region (areas bordering the Río de la Plata). The conflict took place in Uruguay and northeastern Argentina, and on the Río de la Plata. Uruguay's internal troubles, including the longrunning Uruguayan Civil War (La Guerra Grande – "The Great War"), were heavily influential factors leading to the Platine War.

In 1850, the Platine region was politically unstable. Although the Governor of Buenos Aires, Juan Manuel de Rosas, had gained dictatorial control over other Argentine provinces, his rule was plagued by a series of regional rebellions. Meanwhile, Uruguay struggled with its own civil war, which started after gaining independence from the Brazilian Empire in 1828 in the Cisplatine War. Rosas backed the Uruguayan Blanco party in this conflict, and further desired to extend Argentine borders to areas formerly occupied by the Spanish Viceroyalty of the Río de la Plata. This meant asserting control over Uruguay, Paraguay, and Bolivia, which threatened Brazilian interests and sovereignty since the old Spanish Viceroyalty had also included territories which had long been incorporated into the Brazilian province of Rio Grande do Sul.

Brazil actively pursued ways to eliminate the threat from Rosas. In 1851, it allied with the Argentine breakaway provinces of Corrientes and Entre Ríos (led by Justo José de Urquiza), and the anti-Rosas Colorado party in Uruguay. Brazil next secured the south-western flank by signing defensive alliances with Paraguay and Bolivia. Faced with an offensive alliance against his regime, Rosas declared war on Brazil.

Allied forces first advanced into Uruguayan territory, defeating Rosas' Blanco party supporters led by Manuel Oribe. Afterwards, the Allied army was divided, with the main arm advancing by land to engage Rosas' main defenses and the other launching a seaborne assault directed at Buenos Aires.

The Platine War ended in 1852 with the Allied victory at the Battle of Caseros, for some time establishing Brazilian hegemony over much of South America. The war ushered in a period of economic and political stability in the Empire of Brazil. With Rosas gone, Argentina began a political process which would result in a more unified state. However, the end of the Platine war did not completely resolve issues within the Platine region. Turmoil continued in subsequent years, with internal disputes among political factions in Uruguay, a long civil war in Argentina, and an emergent Paraguay asserting its claims. Two more major international wars followed during the next two decades, sparked by territorial ambitions and conflicts over influence.

## Background

### Rosas rule in Argentina

Don Juan Manuel de Rosas became governor of Buenos Aires after the brief period of anarchy following the end of the Cisplatine War in 1828. In theory, Rosas only held as much power as governors of the other Argentine provinces, but in reality, he ruled over the entire Argentine Confederation, as the country was then known. Although he was one of the Federalists, a faction which demanded greater provincial autonomy, in practice Rosas exercised control over the other provinces and become the virtual dictator of Argentina. During his 20-year government, the country witnessed the resurgence of armed conflicts between the Unitarians (his rival political faction) and the Federalists.

Rosas desired to recreate the former Viceroyalty of the Río de la Plata. He aimed to build a powerful, republican state with Argentina placed at the center. The defunct Viceroyalty had shattered into several separate nations following the Argentine War of Independence at the beginning of the 19th century. To achieve reunification, the Argentine government needed to annex the three neighboring countries – Bolivia, Uruguay and Paraguay, as well as to incorporate a portion of the southern region of Brazil. Rosas first had to gather allies across the region who shared his vision. In some instances, this meant that he had to become involved in the internal politics of neighboring countries, backing those sympathetic to union with Argentina, and occasionally even financing rebellions and wars.

### Paraguay

Paraguay considered itself a sovereign nation since 1811, but it was not recognized as such by any other nation. Argentina viewed it as a rebellious province. The Paraguayan dictator José Gaspar Rodríguez de Francia decided that the best way to maintain his own rule and the independence of Paraguay from Argentina was to isolate the country from contacts with the outside world. It was for this reason that, up until 1840, Paraguay had avoided establishing diplomatic relations with other nations. With the death of Francia, this policy began to shift, and his successor Don Carlos Antonio López signed two treaties in July 1841. These were the "Friendship, Commerce and Navigation" and "Limits" agreements made with the Argentine province of Corrientes, which itself had broken away from Argentina under Rosas. Meanwhile, Rosas increasingly put pressure on Paraguay. He continued to refuse to recognize Paraguayan independence and placed a blockade on international traffic to and from Paraguay on the Paraná River.

Paraguay held its own conflict against Juan Manuel de Rosas receiving very substantial support from the Empire of Brazil. President Carlos Antonio López declared war against Rosas and in the first phase of the "Paraguayan front" (1845-1846), Paraguay invaded the Province of Corrientes hoping to cause a rebellion alongside the Argentine dissidents led by Gen. José María Paz, which fared poorly. In the second phase however (1847-1850), the Paraguayans under Gen. Francisco Solano López occupied the Province of Misiones which was under Rosas' rule, resisting in their positions while suffering heavy losses against the counterattacks of the forces of Buenos Aires.

The friendship and alliance between the Republic of Paraguay and the Empire of Brazil remained strong until 1852, when Rosas was finally overthrown. In fact, the Brazilian diplomacy played a key role for the recognition of the Independence of Paraguay. On the other hand, Paraguay gave its full support to Brazil, fighting its own campaign against Buenos Aires. However, in the final phase of the conflict (1851-1852), President Carlos López refused to join forces with the Allied army, keeping a defensive role (as established with Brazil). This was because López was very suspicious of Justo José de Urquiza, former General of Rosas' army who became the leader of the Argentine insurrection.

### Uruguayan conflict

The old Brazilian province of Cisplatina had become the independent Oriental Republic of Uruguay after the Cisplatine War of the 1820s. The country soon was engulfed in a long civil war between its two political parties: the Blancos, led by Don Juan Antonio Lavalleja, and the Colorados, led by Don Fructuoso Rivera.

Lavalleja soon discovered that Rosas in neighbouring Buenos Aires was interested in aiding him financially and militarily. In 1832, Lavalleja began to receive aid from Bento Gonçalves, a soldier and farmer from the Brazilian province of Rio Grande do Sul. Gonçalves had been encouraged by Rosas to rebel against the Brazilian government in 1835, with the ultimate aim of enabling Argentina to annex the province of Rio Grande do Sul. Together, Lavalleja and Gonçalves initiated a military campaign in Uruguay which was characterized by extensive violence and pillage. Gonçalves betrayed Rosas and Lavalleja by becoming an ally of Rivera. Both then invaded Uruguay and overran most of the country outside the environs of the capital, Montevideo. Defeated, the then-Uruguayan president Manuel Oribe, like the betrayed Lavalleja a member of the Blanco party, resigned his position as president and fled to Argentina.

Rosas was determined to restore Argentine suzerainty over Uruguay and take revenge on Gonçalves. A series of interventions resulted. In 1839 an army led by Lavalleja, Oribe and Justo José de Urquiza, (Governor of Entre Rios) was quickly defeated by Rivera. At this point, Lavalleja turned his back on the conflict and played no further part in the civil war. Rosas sent another army of Argentines and Uruguayans in 1845, led by Oribe and Urquiza, and this time defeated Rivera's forces, slaughtering the survivors. Rivera was one of the few who managed to escape, and went into exile in Rio de Janeiro. What remained of Rivera's Colorados held power only in the country's capital Montevideo as Oribe's forces began the Great Siege of Montevideo. The violence in Uruguay escalated, with Oribe's men killing more than 17,000 Uruguayans and 15,000 Argentinians during the conflict.

Oribe's control of nearly all of Uruguay had been secured, enabling him to launch an invasion of southern Brazil, his forces stealing cattle, looting ranches, and liquidating political enemies as they went. More than 188 Brazilian farms were attacked, with 814,000 cattle and 16,950 horses stolen. Local Brazilians independently decided to retaliate, making raids into Uruguay which became known as "Califórnias", in reference to the violence in western North America during California's revolt against Mexico, its brief independence and subsequent annexation by the U.S. As conflict further escalated with the persistent support of Rosas for the Blancos, anarchy spread over wide areas in the region; with a growing threat to trade, the era's two greatest powers, France and the United Kingdom, were induced to declare war on Argentina and impose a blockade on the Río de la Plata. Buenos Aires was repeatedly bombarded by the Anglo-French fleet. The Argentine government was able to mount an effective resistance, however, leading to a peace accord in 1849.

### Empire of Brazil reacts

By the middle of the 19th century, the Empire of Brazil was the richest and most powerful nation in Latin America. It thrived under democratic institutions and a constitutional monarchy and prided itself on the absence of caudillos, dictators and coups d'état which were common across the rest of the continent. During the minority of emperor Pedro II, which led to the regency period in the 1830s, however, there had been several internal rebellions driven by local disputes for power within a few provinces. One of these, the Ragamuffin War, had been led by Gonçalves, as noted above.

For the Brazilian Empire, expansionist plans on the part of powerful, republican Argentina represented an existential threat. It also meant a threat to Brazilian hegemony across its southern borders. A successful Argentine bid to incorporate Paraguay and Uruguay into a reconstituted Viceroyalty of the Río de la Plata (and control of the Platine river network consequently passing into entirely hostile hands) would have threatened to cut communication between the Brazilian province of Mato Grosso and Rio de Janeiro. With river transportation denied, the alternative land routes would require months of travel instead of days. Nor was Brazil keen to share a direct border with Argentina, fearing an increased vulnerability to an invasion by Rosas.

The members of the Brazilian Cabinet could not reach agreement as to how to address the danger posed by Rosas. Some ministers advocated seeking a peaceful resolution at any cost. These feared Brazil was unprepared for war and that a defeat would lead to a situation similar to the chaos following the loss of Cisplatina in the 1820s, which ended in abdication by Dom Pedro I, the Emperor's father. Other ministers took the position that only a military response would eliminate the threat. In 1849, however, Paulino José Soares de Sousa, a member of the pro-war faction and later Viscount of Uruguai, was chosen as the new minister of Foreign Affairs. Soares made clear his intent to deal with Argentina without foreign assistance, announcing that the "Imperial Government does not desire or judge convenient an alliance with France or any other European nation related to the matters in the Platine region. It understands that they must be resolved by the nations which [we] are closely connected with... It will not admit European influence over America." The Empire of Brazil was determined to extend its zone of influence over South America.

The cabinet settled upon a risky alternative to resolve the complicated situation in the Platine region. Instead of undertaking a period of conscription to build up the Imperial Brazilian Army, which would have been costly, the Council decided to rely on the standing army. It sent a contingent south to secure the region. Brazil held an advantage in possessing a powerful and modern navy, along with an experienced professional army hardened by years of internal and external warfare. Up until this point, no other nation in South America possessed true navies or regular armies. Rosas' and Oribe's forces were largely made up of irregular troops on loan from those caudillos who supported them. Even a decade later, Argentina could only field an army of 6,000 men. Brazil also decided to adopt Rosas' own tactics by financing his opponents to weaken him both internally and externally.

### Alliance against Rosas

The Brazilian government set about creating a regional alliance against Rosas, sending a delegation to the region led by Honório Hermeto Carneiro Leão (later the Marquis of Paraná), who held plenipotentiary authority. He was assisted by José Maria da Silva Paranhos (later the Viscount of Rio Branco). Brazil signed a treaty with Bolivia in which Bolivia agreed to strengthen its border defenses to deter any attack by Rosas, though it declined to contribute troops to a war with Argentina.

Isolationist Paraguay was more difficult to win over. Brazil made the initial overtures, becoming the first country to formally recognise Paraguayan independence in 1844. This soon led to the establishment of excellent diplomatic relations. The Brazilian ambassador in Paraguay, Pimenta Bueno, became a private councilor to President Carlos Antonio López. A defensive alliance was signed on 25 December 1850 between Brazil and Paraguay, in which López agreed to supply the Empire with horses for its army. But Paraguay refused to contribute troops to fight Rosas, believing that Justo José de Urquiza (who had invaded Uruguay in 1839 and in 1845), the ruler of the Argentine Entre Ríos province, secretly wished to annex Paraguay.

Brazil's involvement in the Uruguayan civil war also began to deepen. Luís Alves de Lima e Silva, the Count of Caxias, assumed the presidency (governorship) of Rio Grande do Sul and the command of the four Brazilian Army divisions headquartered in the province. Beginning in 1849, the Imperial government directly assisted the besieged Uruguayan government of Colorados in Montevideo, and on 6 September 1850 the Uruguayan representative Andres Lamas signed an agreement with Irineu Evangelista de Sousa to transfer money to the Montevideo government through his bank. On 16 March 1851, the Empire of Brazil openly declared its support of Colorado government of Uruguay against Oribe, something it had been covertly doing for more than two years. This did not please the Argentine government, which began mobilizing for war.

Brazil had also been searching for support against Rosas inside Argentina, with some success. On 1 May 1851, the province of Entre Ríos, still governed by Urquiza, declared to Rosas that "it is the will of its people to re-assume the entire exercise of its own sovereignty and power which had been delegated to the governor of Buenos Aires." It was followed by the province of Corrientes, governed by Benjamín Virasoro, which sent the same message. Brazil encouraged and financially supported both uprisings. One of the reasons for Urquiza's betrayal of Rosas was a long-running rivalry. Rosas had tried to remove him from power several times since 1845, suspecting that the caudillo was nurturing designs for his overthrow. This was the trigger for military intervention, and Brazil sent a naval force to the Platine region, basing it near the port of Montevideo. The British Rear admiral John Pascoe Grenfell, a veteran of the Brazilian War of Independence and of the Cisplatine War, was appointed to lead the fleet, which reached Montevideo on 4 May 1851. His command included 1 frigate, 7 corvettes, 3 brigs and 6 steamships. The Brazilian Armada had a total of 59 vessels of various types in 1851, including 36 armed sailing ships, 10 armed steamships, 7 unarmed sailing ships and 6 sailing transports.

Uruguay, Brazil and the Argentine provinces of Entre Rios and Corrientes joined in an offensive Alliance against Rosas on 29 May 1851. The text of the treaty declared that the objective was to protect Uruguayan independence, pacify its territory, and expel Oribe's forces. Urquiza would command the Argentine forces and Eugenio Garzón would lead the Colorado Uruguayans, with both receiving financial and military aid from the Empire of Brazil.

This was followed on 2 August 1851 by landings of the first Brazilian detachments in Uruguay, consisting of approximately 300 soldiers of the 6th Battalion of Skirmishers sent to protect Fuerte del Cerro (Cerro Fort). In response, Rosas declared war against Brazil on 18 August 1851.

According to Herrera y Obes, Rosas would have said, as war approached: "poor Brazilians, I'm going to make of their Emperor my butler".

## Stage 1 – Allied invasion of Uruguay

The Count of Caxias led a Brazilian army of 16,200 professional soldiers across the border between Rio Grande do Sul and Uruguay on 4 September 1851. His force consisted of four divisions, with 6,500 infantrymen, 8,900 cavalrymen, 800 artillerymen and 26 cannons, a little under half the total Brazilian army (37,000 men); while another 4,000 of his men remained in Brazil to protect its border.

The Brazilian Army entered Uruguay in three groups: the main force, consisting of the 1st and 2nd divisions left from Santana do Livramento—around 12,000 men under Caxias's personal command. The second force, under the command of Colonel David Canabarro departed from Quaraim, comprising the 4th division, protecting Caxias' right flank. The third force, the 3rd Division under Brigadier General José Fernandes, left from Jaguarão, protecting Caxias' left. Canabarro's 4th Division joined Caxias's troops shortly after arriving at the Uruguayan town of Frutuoso, the combined force then joining up with Fernandes just before reaching Montevideo.

### Defeat of Oribe

Meanwhile, the troops of Urquiza and Eugenio Garzón had surrounded the army of Oribe near Montevideo. Their forces numbered roughly 15,000 men, almost double Oribe's 8,500. Realising that the Brazilians were approaching and knowing that there was no hope of victory, Oribe ordered his troops to surrender without a fight on 19 October, and retreated into seclusion on his farm in Paso del Molino.

The Brazilian fleet, with their ships scattered throughout the River Plate and tributaries, prevented the defeated army of Oribe from escaping into Argentina. Urquiza suggested to Grenfell that they should simply kill the resulting prisoners of war, but Grenfell refused to harm any of them. Instead, Oribe's Argentine soldiers were incorporated into the army of Urquiza, and the Uruguayans into Garzon's.

The Brazilian army easily took the remaining Blanco Uruguayan territory, fending off remnants of Oribe's troops who attacked its flanks in several skirmishes. On 21 November the representatives of Brazil, Uruguay, Entre Ríos and Corrientes then formed another alliance in Montevideo with the objective of "freeing the Argentine people from the oppression it suffers under the tyrant rule of Governor Rosas".

## Stage 2 – Allied invasion of Argentina

### Allied army advance

Shortly after the surrender of Oribe, the Allied forces split into two groups, the plan being for one force to maneuver upriver and sweep down on Buenos Aires from Santa Fe, while the other would make a landing at the port of Buenos Aires itself. The first of these groups was composed of Uruguayan and Argentine troops, along with the 1st Division of the Brazilian Army under Brigadier General Manuel Marques de Sousa (later the Count of Porto Alegre). It was initially based in the town of Colonia del Sacramento in the south of Uruguay across the Río de la Plata estuary from the city of Buenos Aires.

On 17 December 1851, a squadron of Brazilian ships, consisting of four steamships, three corvettes and one brig under the command of Grenfell, forced a passage of the Paraná River in the battle which became known as the Passage of the Tonelero. The Argentines had formed a powerful defensive line at The Tonelero Pass, near the cliffs of Acevedo, protected by 16 pieces of artillery and 2,000 riflemen under the command of general Lucio Norberto Mansilla. The Argentine troops exchanged fire with the Brazilian warships but were unable to prevent them from progressing upriver. The following day, the Brazilian ships returned and broke their way through the Tonelero's defences, carrying the remaining troops of Marques de Sousa's Brazilian division upstream towards Gualeguaichu. This second influx of ships caused Mansilla and his soldiers to withdraw in chaos, abandoning their artillery, believing that the Allies were intending to land and attack their positions from the rear.

The Allied army continued to make its way to the assembly point at Gualeguaichu. Urquiza and his cavalry traveled overland from Montevideo, while the infantry and artillery were carried by Brazilian warships up the Uruguay River. After meeting up, they marched west until they reached the city of Diamante on the east side of the Paraná River in the middle of December 1851. Eugenio Garzón and the Uruguayan troops were taken from Montevideo up to Potrero Perez by Brazilian warships and continued on foot until arriving at Diamante on 30 December 1851, when all the Allied forces were finally reunited.

From Diamante contingents were ferried to the other side of the Paraná River, landing at Santa Fe. The Confederate Argentine troops in the region ran away without offering any resistance. The Allied Army, or the "Grand Army of South America" as it was officially called by Urquiza, marched on towards Buenos Aires.

Meanwhile, the second force, comprising the majority of the Brazilian troops (about 12,000 men) under the command of Caxias, had remained in Colonia del Sacramento. The Brazilian commander took the steamship Dom Afonso (named in honor of the late Prince Afonso) and entered the port at Buenos Aires to select the best place to disembark his troops. He expected to have to defeat the Argentine flotilla anchored there, but the force did nothing to stop him and he safely returned to Sacramento to plan his assault. The attack was aborted, however, when news arrived of the Allied victory at Caseros.

### Rosas defeated

The Allied army had been advancing on the Argentine capital of Buenos Aires by land, while the Brazilian Army commanded by Caxias planned a supporting attack by sea. On 29 January, the Allied vanguard defeated a force of 4,000 Argentines led by two colonels which General Ángel Pacheco had sent to slow down the advance at the Battle of Alvarez Field. Pacheco retreated. Two days later, troops under his personal command were defeated at the Battle of Marques Bridge by two allied divisions. On 1 February 1852, the Allied troops encamped approximately nine kilometers from Buenos Aires. The next day a brief skirmish between the vanguards of both armies ended with a retreat by the Argentines.

On 3 February the Allied army encountered the main Argentine force commanded by Rosas himself. On paper, the two sides were well-matched. The Allies included 20,000 Argentines, 2,000 Uruguayans, 4,000 Brazilian elite troops totalling 26,000 men and 45 cannon (16,000 cavalrymen, 9,000 infantrymen and 1,000 artillerymen). On the Argentine side, Rosas had 15,000 cavalrymen, 10,000 infantrymen and 1,000 artillerymen, totalling 26,000 men and 60 cannon. Rosas had been able to select the best positions for his army, choosing the high ground on the slopes of a hill at Caseros, which lay on the other side of the Morón creek. His headquarters were in a mansion at the top of Caseros.

The Allied commanders were Marques de Sousa, Manuel Luis Osório (later the Marquis of Erval), Jose Maria Pirán, Jose Miguel Galán (who replaced Garzón after his unexpected death in December 1851), Urquiza, and the future Argentine presidents Bartolomé Mitre and Domingo Sarmiento—the latter both leading the Argentine Unitarians. These men formed a War Council, and gave orders to commence the attack. Almost immediately the forward units of the two armies began to engage in battle.

The Battle of Caseros resulted in a decisive victory for the Allies. Although they started with the inferior position on the battlefield, the Allied soldiers managed to annihilate Rosas' troops in a fight that lasted for most of the day. A few minutes before the Allied forces reached Rosas' headquarters, the Argentine dictator escaped the battlefield. Disguised as a sailor, he sought out Robert Gore, the British ambassador in Buenos Aires, and requested asylum. The ambassador agreed to have Rosas and his daughter Manuelita taken to the United Kingdom, where he would spend the last twenty years of his life. The official report stated that 600 men on the Allied side had been wounded or died, while the Argentine losses were 1,400 killed or wounded and 7,000 captured. Given the duration and scale of the battle, however, this may be an underestimate.

To mark their victory, the Allied troops marched in triumph through the streets of Buenos Aires. The parades included the Brazilian Army, which insisted that their triumphal procession take place on 20 February, to mark payback for the defeat it had suffered at the Battle of Ituzaingó twenty five years before on that date. The population of Buenos Aires was said to have looked on silently with a combination of shame and hostility as the Brazilians passed.

## Aftermath

### Brazil

The triumph in Caseros was a pivotal military victory for Brazil. The independence of Paraguay and Uruguay was secured, and the planned Argentine invasion of Rio Grande do Sul was blocked. In a period of three years, the Empire of Brazil had destroyed any possibility of reconstituting a state encompassing the territories of the old Viceroyalty of the Río de la Plata, a goal cherished by many in Argentina since independence. Brazil's army and fleet had accomplished what the United Kingdom and France, the great powers of that time, had not achieved through interventions by their powerful navies. This represented a watershed for the history of the region, ushering in Brazilian hegemony over the Platine region and, according to Brazilian historian J. F. Maya Pedrosa, the rest of South America as well. The War of the Triple Alliance eighteen years later would only be a confirmation of Brazilian dominance. The country came out of the conflict with its monarchy strengthened and the cessation of internal revolts, including in the province of Rio Grande do Sul, entering into a period of economic and cultural prosperity.

### Argentina

Soon after the Battle of Caseros the San Nicolás Agreement was signed. It was meant to comply with the constitutional mandate of the Federal pact that presided over the Argentine Confederation, convening a Constitutional Assembly to meet in Santa Fe. This agreement was not accepted by the province of Buenos Aires, since it reduced its influence and power over the other provinces. Following the revolution of 11 September 1852, Buenos Aires seceded from the confederation, thus dividing Argentina into two rival, independent states which fought to establish dominance. On the one side were the Federalists of the Argentine Confederation, led by Justo José de Urquiza. On the other, the State of Buenos Aires. The civil war was only brought to an end with the decisive victory of Buenos Aires over the Confederation at the 1861 Battle of Pavón. The bonaerense liberal leader Bartolomé Mitre was elected the first President of a united Argentine Republic in 1862.

### Paraguay and Uruguay

With the opening of the Platine rivers, Paraguay found it possible to contract with European technicians and Brazilian specialists to aid in its development. Unhindered access to the outside world also enabled it to import more advanced military technology. During the greater part of the 1850s, the dictator Carlos López harassed Brazilian vessels attempting to freely navigate the Paraguay River. López feared that the province of Mato Grosso might become a base from which an invasion from Brazil could be launched. This dispute was also leverage with the Imperial government for acceptance of his territorial demands in the region. The nation also experienced difficulties in delimiting its borders with Argentina which wanted the whole Gran Chaco region: a demand which Paraguay could not accept, as this would entail surrendering more than half of its national territory.

The end of the Platine War did not bring a halt to conflict in the region. Peace remained out of reach in Uruguay, which remained unstable and in a state of constant crisis due to continuing internecine strife between the Blancos and Colorados. Border disputes, power struggles among diverse regional factions, and attempts to establish regional and internal influence would eventually spark the Uruguayan War as well as the later Paraguayan War.

## Endnotes

1851–1852 war between Argentina and Brazil


Discovery of nuclear fission

Nuclear fission was discovered in December 1938 by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Fission is a nuclear reaction or radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei and often other particles. The fission process often produces gamma rays and releases a very large amount of energy, even by the energetic standards of radioactive decay. Scientists already knew about alpha decay and beta decay, but fission assumed great importance because the discovery that a nuclear chain reaction was possible led to the development of nuclear power and nuclear weapons. Hahn was awarded the 1944 Nobel Prize in Chemistry for the discovery of nuclear fission.

Hahn and Strassmann at the Kaiser Wilhelm Institute for Chemistry in Berlin bombarded uranium with slow neutrons and discovered that barium had been produced. Hahn suggested a bursting of the nucleus, but he was unsure of what the physical basis for the results were. They reported their findings by mail to Meitner in Sweden, who a few months earlier had fled Nazi Germany. Meitner and her nephew Frisch theorised, and then proved, that the uranium nucleus had been split and published their findings in Nature. Meitner calculated that the energy released by each disintegration was approximately 200 megaelectronvolts, and Frisch observed this. By analogy with the division of biological cells, he named the process "fission".

The discovery came after forty years of investigation into the nature and properties of radioactivity and radioactive substances. The discovery of the neutron by James Chadwick in 1932 created a new means of nuclear transmutation. Enrico Fermi and his colleagues in Rome studied the results of bombarding uranium with neutrons, and Fermi concluded that his experiments had created new elements with 93 and 94 protons, which his group dubbed ausenium and hesperium. Fermi won the 1938 Nobel Prize in Physics for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". However, not everyone was convinced by Fermi's analysis of his results. Ida Noddack suggested that instead of creating a new, heavier element 93, it was conceivable that the nucleus had broken up into large fragments, and Aristid von Grosse suggested that what Fermi's group had found was an isotope of protactinium.

This spurred Hahn and Meitner, the discoverers of the most stable isotope of protactinium, to conduct a four-year-long investigation into the process with their colleague Strassmann. After much hard work and many discoveries, they determined that what they were observing was fission, and that the new elements that Fermi had found were fission products. Their work overturned long-held beliefs in physics and paved the way for the discovery of the real elements 93 (neptunium) and 94 (plutonium), for the discovery of fission in other elements, and for the determination of the role of the uranium-235 isotope in that of uranium. Niels Bohr and John Wheeler reworked the liquid drop model to explain the mechanism of fission.

## Background

### Radioactivity

In the last years of the 19th century, scientists frequently experimented with the cathode-ray tube, which by then had become a standard piece of laboratory equipment. A common practice was to aim the cathode rays at various substances and to see what happened. Wilhelm Röntgen had a screen coated with barium platinocyanide that would fluoresce when exposed to cathode rays. On 8 November 1895, he noticed that even though his cathode-ray tube was not pointed at his screen, which was covered in black cardboard, the screen still fluoresced. He soon became convinced that he had discovered a new type of rays, which are today called X-rays. The following year Henri Becquerel was experimenting with fluorescent uranium salts, and wondered if they too might produce X-rays. On 1 March 1896 he discovered that they did indeed produce rays, but of a different kind, and even when the uranium salt was kept in a dark drawer, it still made an intense image on an X-ray plate, indicating that the rays came from within, and did not require an external energy source.

Unlike Röntgen's discovery, which was the object of widespread curiosity from scientists and lay people alike for the ability of X-rays to make visible the bones within the human body, Becquerel's discovery made little impact at the time, and Becquerel himself soon moved on to other research. Marie Curie tested samples of as many elements and minerals as she could find for signs of Becquerel rays, and in April 1898 also found them in thorium. She gave the phenomenon the name "radioactivity". Along with Pierre Curie and Gustave Bémont, she began investigating pitchblende, a uranium-bearing ore, which was found to be more radioactive than the uranium it contained. This indicated the existence of additional radioactive elements. One was chemically akin to bismuth, but strongly radioactive, and in July 1898 they published a paper in which they concluded that it was a new element, which they named "polonium". The other was chemically like barium, and in a December 1898 paper they announced the discovery of a second hitherto unknown element, which they called "radium". Convincing the scientific community was another matter. Separating radium from the barium in the ore proved very difficult. It took three years for them to produce a tenth of a gram of radium chloride, and they never did manage to isolate polonium.

In 1898, Ernest Rutherford noted that thorium gave off a radioactive gas. In examining the radiation, he classified Becquerel radiation into two types, which he called α (alpha) and β (beta) radiation. Subsequently, Paul Villard discovered a third type of Becquerel radiation which, following Rutherford's scheme, were called "gamma rays", and Curie noted that radium also produced a radioactive gas. Identifying the gas chemically proved frustrating; Rutherford and Frederick Soddy found it to be inert, much like argon. It later came to be known as radon. Rutherford identified beta rays as cathode rays (electrons), and hypothesised—and in 1909 with Thomas Royds proved—that alpha particles were helium nuclei. Observing the radioactive disintegration of elements, Rutherford and Soddy classified the radioactive products according to their characteristic rates of decay, introducing the concept of a half-life. In 1903, Soddy and Margaret Todd applied the term "isotope" to atoms that were chemically and spectroscopically identical but had different radioactive half-lives. Rutherford proposed a model of the atom in which a very small, dense and positively charged nucleus of protons was surrounded by orbiting, negatively charged electrons (the Rutherford model). Niels Bohr improved upon this in 1913 by reconciling it with the quantum behaviour of electrons (the Bohr model).

### Protactinium

Soddy and Kasimir Fajans independently observed in 1913 that alpha decay caused atoms to shift down two places in the periodic table, while the loss of two beta particles restored it to its original position. In the resulting reorganisation of the periodic table, radium was placed in group II, actinium in group III, thorium in group IV and uranium in group VI. This left a gap between thorium and uranium. Soddy predicted that this unknown element, which he referred to (after Dmitri Mendeleev) as "ekatantalium", would be an alpha emitter with chemical properties similar to tantalium (now known as tantalum). It was not long before Fajans and Oswald Helmuth Göhring discovered it as a decay product of a beta-emitting product of thorium. Based on the radioactive displacement law of Fajans and Soddy, this was an isotope of the missing element, which they named "brevium" after its short half-life. However, it was a beta emitter, and therefore could not be the mother isotope of actinium. This had to be another isotope.

Two scientists at the Kaiser Wilhelm Institute (KWI) in Berlin-Dahlem took up the challenge of finding the missing isotope. Otto Hahn had graduated from the University of Marburg as an organic chemist, but had been a post-doctoral researcher at University College London under Sir William Ramsay, and under Rutherford at McGill University, where he had studied radioactive isotopes. In 1906, he returned to Germany, where he became an assistant to Emil Fischer at the University of Berlin. At McGill he had become accustomed to working closely with a physicist, so he teamed up with Lise Meitner, who had received her doctorate from the University of Vienna in 1906, and had then moved to Berlin to study physics under Max Planck at the Friedrich-Wilhelms-Universität. Meitner found Hahn, who was her own age, less intimidating than older, more distinguished colleagues. Hahn and Meitner moved to the recently established Kaiser Wilhelm Institute for Chemistry in 1913, and by 1920 had become the heads of their own laboratories there, with their own students, research programs and equipment. The new laboratories offered new opportunities, as the old ones had become too contaminated with radioactive substances to investigate feebly radioactive substances. They developed a new technique for separating the tantalum group from pitchblende, which they hoped would speed the isolation of the new isotope.

The work was interrupted by the outbreak of the First World War in 1914. Hahn was called up into the German Army, and Meitner became a volunteer radiographer in Austrian Army hospitals. She returned to the Kaiser Wilhelm Institute in October 1916. Hahn joined the new gas command unit at Imperial Headquarters in Berlin in December 1916 after travelling between the western and eastern fronts, Berlin and Leverkusen between the summer of 1914 and late 1916.

Most of the students, laboratory assistants and technicians had been called up, so Hahn, who was stationed in Berlin between January and September of 1917, and Meitner had to do everything themselves. By December 1917 she was able to isolate the substance, and after further work were able to prove that it was indeed the missing isotope. Meitner submitted her and Hahn ́s findings for publication in March 1918 to the scientific paper Physikalischen Zeitschrift under the title Die Muttersubstanz des Actiniums; ein neues radioaktives Element von langer Lebensdauer.

Although Fajans and Göhring had been the first to discover the element, custom required that an element was represented by its longest-lived and most abundant isotope, and brevium did not seem appropriate. Fajans agreed to Meitner and Hahn naming the element protactinium, and assigning it the chemical symbol Pa. In June 1918, Soddy and John Cranston announced that they had extracted a sample of the isotope, but unlike Hahn and Meitner were unable to describe its characteristics. They acknowledged Hahn ́s and Meitner's priority, and agreed to the name. The connection to uranium remained a mystery, as neither of the known isotopes of uranium decayed into protactinium. It remained unsolved until uranium-235 was discovered in 1929.

For their discovery Hahn and Meitner were repeatedly nominated for the Nobel Prize in Chemistry in the 1920s by several scientists, among them Max Planck, Heinrich Goldschmidt, and Fajans himself. In 1949, the International Union of Pure and Applied Chemistry (IUPAC) named the new element definitively protactinium, and confirmed Hahn and Meitner as discoverers.

## Transmutation

Patrick Blackett was able to accomplish nuclear transmutation of nitrogen into oxygen in 1925, using alpha particles directed at nitrogen. In modern notation for the atomic nuclei, the reaction was:

14
7N + 4
2He → 17
8O + p

This was the first observation of a nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. A fully artificial nuclear reaction and nuclear transmutation was achieved in April 1932 by Ernest Walton and John Cockcroft, who used artificially accelerated protons against lithium, to break this nucleus into two alpha particles. The feat was popularly known as "splitting the atom", but was not nuclear fission; as it was not the result of initiating an internal radioactive decay process. Just a few weeks before Cockcroft and Walton's feat, another scientist at the Cavendish Laboratory, James Chadwick, discovered the neutron, using an ingenious device made with sealing wax, through the reaction of beryllium with alpha particles:

9
4Be + 4
2He → 12
6C + n

Irène Curie and Frédéric Joliot irradiated aluminium foil with alpha particles and found that this results in a short-lived radioactive isotope of phosphorus with a half-life of around three minutes:

27
13Al + 4
2He → 30
15P + n

which then decays to a stable isotope of silicon

30
15P → 30
14Si + e+

They noted that radioactivity continued after the neutron emissions ceased. Not only had they discovered a new form of radioactive decay in the form of positron emission, they had transmuted an element into a hitherto unknown radioactive isotope of another, thereby inducing radioactivity where there had been none before. Radiochemistry was now no longer confined to certain heavy elements, but extended to the entire periodic table.

Chadwick noted that being electrically neutral, neutrons would be able to penetrate the nucleus more easily than protons or alpha particles. Enrico Fermi and his colleagues in Rome—Edoardo Amaldi, Oscar D'Agostino, Franco Rasetti and Emilio Segrè—picked up on this idea. Rasetti visited Meitner's laboratory in 1931, and again in 1932 after Chadwick's discovery of the neutron. Meitner showed him how to prepare a polonium-beryllium neutron source. On returning to Rome, Rasetti built Geiger counters and a cloud chamber modelled after Meitner's. Fermi initially intended to use polonium as a source of alpha particles, as Chadwick and Curie had done. Radon was a stronger source of alpha particles than polonium, but it also emitted beta and gamma rays, which played havoc with the detection equipment in the laboratory. But Rasetti went on his Easter vacation without preparing the polonium-beryllium source, and Fermi realised that since he was interested in the products of the reaction, he could irradiate his sample in one laboratory and test it in another down the hall. The neutron source was easy to prepare by mixing with powdered beryllium in a sealed capsule. Moreover, radon was easily obtained; Giulio Cesare Trabacchi had more than a gram of radium and was happy to supply Fermi with radon. With a half-life of only 3.82 days it would only go to waste otherwise, and the radium continually produced more.

Working in assembly-line fashion, they started by irradiating water, and then progressed up the periodic table through lithium, beryllium, boron and carbon, without inducing any radioactivity. When they got to aluminium and then fluorine, they had their first successes. Induced radioactivity was ultimately found through the neutron bombardment of 22 different elements. Meitner was one of the select group of physicists to whom Fermi mailed advance copies of his papers, and she was able to report that she had verified his findings with respect to aluminium, silicon, phosphorus, copper and zinc. When a new copy of La Ricerca Scientifica arrived at the Niels Bohr's Institute for Theoretical Physics at the University of Copenhagen, her nephew, Otto Frisch, as the only physicist there who could read Italian, found himself in demand from colleagues wanting a translation. The Rome group had no samples of the rare earth metals, but at Bohr's institute George de Hevesy had a complete set of their oxides that had been given to him by Auergesellschaft, so de Hevesy and Hilde Levi carried out the process with them.

When the Rome group reached uranium, they had a problem: the radioactivity of natural uranium was almost as great as that of their neutron source. What they observed was a complex mixture of half-lives. Following the displacement law, they checked for the presence of lead, bismuth, radium, actinium, thorium and protactinium (skipping the elements whose chemical properties were unknown), and (correctly) found no indication of any of them. Fermi noted three types of reactions were caused by neutron irradiation: emission of an alpha particle (n, α); proton emission (n, p); and gamma emission (n, γ). Invariably, the new isotopes decayed by beta emission, which caused elements to move up the periodic table.

Based on the periodic table of the time, Fermi believed that element 93 was ekarhenium—the element below rhenium—with characteristics similar to manganese and rhenium. Such an element was found, and Fermi tentatively concluded that his experiments had created new elements with 93 and 94 protons, which he dubbed ausonium and hesperium. The results were published in Nature in June 1934. However, in this paper Fermi cautioned that "a careful search for such heavy particles has not yet been carried out, as they require for their observation that the active product should be in the form of a very thin layer. It seems therefore at present premature to form any definite hypothesis on the chain of disintegrations involved." In retrospect, what they had detected was indeed an unknown rhenium-like element, technetium, which lies between manganese and rhenium on the periodic table.

Leo Szilard and Thomas A. Chalmers reported that neutrons generated by gamma rays acting on beryllium were captured by iodine, a reaction that Fermi had also noted. When Meitner repeated their experiment, she found that neutrons from the gamma-beryllium sources were captured by heavy elements like iodine, silver and gold, but not by lighter ones like sodium, aluminium and silicon. She concluded that slow neutrons were more likely to be captured than fast ones, a finding she reported to Naturwissenschaften in October 1934. Everyone had been thinking that energetic neutrons were required, as was the case with alpha particles and protons, but that was required to overcome the Coulomb barrier; the neutrally charged neutrons were more likely to be captured by the nucleus if they spent more time in its vicinity. A few days later, Fermi considered a curiosity that his group had noted: uranium seemed to react differently in different parts of the laboratory; neutron irradiation conducted on a wooden table induced more radioactivity than on a marble table in the same room. Fermi thought about this and tried placing a piece of paraffin wax between the neutron source and the uranium. This resulted in a dramatic increase in activity. He reasoned that the neutrons had been slowed by collisions with hydrogen atoms in the paraffin and wood. The departure of D'Agostino meant that the Rome group no longer had a chemist, and the subsequent loss of Rasetti and Segrè reduced the group to just Fermi and Amaldi, who abandoned the research into transmutation to concentrate on exploring the physics of slow neutrons.

The current model of the nucleus in 1934 was the liquid drop model first proposed by George Gamow in 1930. His simple and elegant model was refined and developed by Carl Friedrich von Weizsäcker and, after the discovery of the neutron, by Werner Heisenberg in 1935 and Niels Bohr in 1936, it agreed closely with observations. In the model, the nucleons were held together in the smallest possible volume (a sphere) by the strong nuclear force, which was capable of overcoming the longer ranged Coulomb electrical repulsion between the protons. The model remained in use for certain applications into the 21st century, when it attracted the attention of mathematicians interested in its properties, but in its 1934 form it confirmed what physicists thought they already knew: that nuclei were static, and that the odds of a collision chipping off more than an alpha particle were practically zero.

## Discovery

### Objections

Fermi won the 1938 Nobel Prize in Physics for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". However, not everyone was convinced by Fermi's analysis of his results. Ida Noddack suggested in September 1934 that instead of creating a new, heavier element 93, that:

> One could assume equally well that when neutrons are used to produce nuclear disintegrations, some distinctly new nuclear reactions take place which have not been observed previously with proton or alpha-particle bombardment of atomic nuclei. In the past one has found that transmutations of nuclei only take place with the emission of electrons, protons, or helium nuclei, so that the heavy elements change their mass only a small amount to produce near neighbouring elements. When heavy nuclei are bombarded by neutrons, it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbours of the irradiated element.

Noddack's article was read by Fermi's team in Rome, Curie and Joliot in Paris, and Meitner and Hahn in Berlin. However, the quoted objection comes some distance down, and is but one of several gaps she noted in Fermi's claim. Bohr's liquid drop model had not yet been formulated, so there was no theoretical way to calculate whether it was physically possible for the uranium atoms to break into large pieces. Noddack and her husband, Walter Noddack, were renowned chemists who had been nominated for the Nobel Prize in Chemistry for the discovery of rhenium, although at the time they were also embroiled in a controversy over the discovery of element 43, which they called "masurium". The discovery of technetium by Emilio Segrè and Carlo Perrier put an end to their claim, but did not occur until 1937. It is unlikely that Meitner or Curie had any prejudice against Noddack because of her sex, but Meitner was not afraid to tell Hahn Hähnchen, von Physik verstehst Du Nichts ("Hahn dear, of physics you understand nothing"). The same attitude carried over to Noddack, who did not propose an alternative nuclear model, nor conduct experiments to support her claim. Although Noddack was a renowned analytical chemist, she lacked the background in physics to appreciate the enormity of what she was proposing.

Noddack was not the only critic of Fermi's claim. Aristid von Grosse suggested that what Fermi had found was an isotope of protactinium. Meitner was eager to investigate Fermi's results, but she recognised that a highly skilled chemist was required, and she wanted the best one she knew: Hahn, although they had not collaborated for many years. Initially, Hahn was not interested, but von Grosse's mention of protactinium changed his mind. "The only question", Hahn later wrote, "seemed to be whether Fermi had found isotopes of transuranian elements, or isotopes of the next-lower element, protactinium. At that time Lise Meitner and I decided to repeat Fermi's experiments in order to find out whether the 13-minute isotope was a protactinium isotope or not. It was a logical decision, having been the discoverers of protactinium."

Hahn and Meitner were joined by Fritz Strassmann. Strassmann had received his doctorate in analytical chemistry from the Technical University of Hannover in 1929, and had come to the Kaiser Wilhelm Institute for Chemistry to study under Hahn, believing that this would improve his employment prospects. He enjoyed the work and the people so much that he stayed on after his stipend expired in 1932. After the Nazi Party came to power in Germany in 1933, he declined a lucrative offer of employment because it required political training and Nazi Party membership, and he resigned from the Society of German Chemists when it became part of the Nazi German Labour Front. As a result, he could neither work in the chemical industry nor receive his habilitation, which was required to become an independent researcher in Germany. Meitner persuaded Hahn to hire Strassmann using money from the director's special circumstances fund. In 1935, Strassmann became an assistant on half pay. Soon he would be credited as a collaborator on the papers they produced.

The 1933 Law for the Restoration of the Professional Civil Service removed Jewish people from the civil service, which included academia. Meitner never tried to conceal her Jewish descent, but initially was exempt from its impact on multiple grounds: she had been employed before 1914, had served in the military during the World War, was an Austrian rather than a German citizen, and the Kaiser Wilhelm Institute was a government-industry partnership. However, she was dismissed from her adjunct professorship at the University of Berlin on the grounds that her World War I service was not at the front, and she had not completed her habilitation until 1922. Carl Bosch, the director of IG Farben, a major sponsor of the Kaiser Wilhelm Institute for Chemistry, assured Meitner that her position there was safe, and she agreed to stay. Meitner, Hahn and Strassmann drew closer together personally as their anti-Nazi politics increasingly alienated them from the rest of the organisation, but it gave them more time for research, as administration was devolved to Hahn's and Meitner's assistants.

### Research

The Berlin group started by irradiating uranium salt with neutrons from a radon-beryllium source similar to the one that Fermi had used. They dissolved it and added potassium perrhenate, platinum chloride and sodium hydroxide. What remained was then acidified with hydrogen sulphide, resulting in platinum sulphide and rhenium sulphide precipitation. Fermi had noted four radioactive isotopes with the longest-lived having 13- and 90-minute half-lives, and these were detected in the precipitate. The Berlin group then tested for protactinium by adding protactinium-234 to the solution. When this was precipitated, it was found to be separated from the 13- and 90-minute half-life isotopes, demonstrating that von Grosse was incorrect, and they were not isotopes of protactinium. Moreover, the chemical reactions involved ruled out all elements from mercury and above on the periodic table. They were able to precipitate the 90-minute activity with osmium sulphide and the 13-minute one with rhenium sulphide, which ruled out them being isotopes of the same element. All this provided strong evidence that they were indeed transuranium elements, with chemical properties similar to osmium and rhenium.

Fermi had also reported that fast and slow neutrons had produced different activities. This indicated that more than one reaction was taking place. When the Berlin group could not replicate the Rome group's findings, they commenced their own research into the effects of fast and slow neutrons. To minimise radioactive contamination if there were an accident, different phases were carried out in different rooms, all in Meitner's section on the ground floor of the Kaiser Wilhelm Institute. Neutron irradiation was carried out in one laboratory, chemical separation in another, and measurements were conducted in a third. The equipment they used was simple and mostly hand made.

By March 1936, they had identified ten different half-lives, with varying degrees of certainty. To account for them, Meitner had to hypothesise a new (n, 2n) class of reaction and the alpha decay of uranium, neither of which had ever been reported before, and for which physical evidence was lacking. So while Hahn and Strassmann refined their chemical procedures, Meitner devised new experiments to shine more light on the reaction processes. In May 1937, they issued parallel reports, one in Zeitschrift für Physik with Meitner as the principal author, and one in Chemische Berichte with Hahn as the principal author. Hahn concluded his by stating emphatically: Vor allem steht ihre chemische Verschiedenheit von allen bisher bekannten Elementen außerhalb jeder Diskussion ("Above all, their chemical distinction from all previously known elements needs no further discussion.") Meitner was increasingly uncertain. They had now constructed three (n, γ) reactions:

1. 238
    92U + n → 239
    92U (10 seconds) → 239
    93ekaRe (2.2 minutes) → 239
    94ekaOs (59 minutes) → 239
    95ekaIr (66 hours) → 239
    96ekaPt (2.5 hours) → 239
    97ekAu (?)
2. 238
    92U + n → 239
    92U (40 seconds) → 239
    93ekaRe (16 minutes) → 239
    94ekaOs (5.7 hours) → 239
    95ekaIr (?)
3. 238
    92U + n → 239
    92U (23 minutes) → 239
    93ekaRe

Meitner was certain that these had to be (n, γ) reactions, as slow neutrons lacked the energy to chip off protons or alpha particles. She considered the possibility that the reactions were from different isotopes of uranium; three were known: uranium-238, uranium-235 and uranium-234. However, when she calculated the neutron cross section it was too large to be anything other than the most abundant isotope, uranium-238. She concluded that it must be a case of nuclear isomerism, which had been discovered in protactinium by Hahn in 1922. Nuclear isomerism had been given a physical explanation by von Weizsäcker, who had been Meitner's assistant in 1936, but had since taken a position at the Kaiser Wilhelm Institute for Physics. Different nuclear isomers of protactinium had different half-lives, and this could be the case for uranium too, but if so it was somehow being inherited by the daughter and granddaughter products, which seemed to be stretching the argument to breaking point. Then there was the third reaction, an (n, γ) one, which occurred only with slow neutrons. Meitner therefore ended her report on a very different note to Hahn, reporting that: "The process must be neutron capture by uranium-238, which leads to three isomeric nuclei of uranium-239. This result is very difficult to reconcile with current concepts of the nucleus."

After this, the Berlin group moved on to working with thorium, as Strassmann put it, "to recover from, the horror of the work with uranium". However, thorium was not easier to work with than uranium. For a start, it had a decay product, radiothorium (228
90Th) that overwhelmed weaker neutron-induced activity. But Hahn and Meitner had a sample from which they had regularly removed its mother isotope, mesothorium (228
88Ra), over a period of several years, allowing the radiothorium to decay away. Even then, it was still more difficult to work with because its induced decay products from neutron irradiation were isotopes of the same elements produced by thorium's own radioactive decay. What they found was three different decay series, all alpha emitters—a form of decay not found in any other heavy element, and for which Meitner once again had to postulate multiple isomers. They did find an interesting result: these (n, α) decay series occurred simultaneously when the energy of the incident neutrons was less than 2.5 MeV; when they had more, an (n, γ) reaction that formed 233
90Th was favoured.

In Paris, Irene Curie and Pavel Savitch had also set out to replicate Fermi's findings. In collaboration with Hans von Halban and Peter Preiswerk, they irradiated thorium and produced the isotope with a 22-minute half-life that Fermi had noted. In all, Curie's group detected eight different half-lives in their irradiated thorium. Curie and Savitch detected a radioactive substance with a 3.5-hour half-life. The Paris group proposed that it might be an isotope of thorium. Meitner asked Strassmann, who was now doing most of the chemistry work, to check. He detected no sign of thorium. Meitner wrote to Curie with their results, and suggested a quiet retraction. Nonetheless, Curie persisted. They investigated the chemistry, and found that the 3.5-hour activity was coming from something that seemed to be chemically similar to lanthanum (which in fact it was), which they attempted unsuccessfully to isolate with a fractional crystallization process. (It is possible that their precipitate was contaminated with yttrium, which is chemically similar.) By using Geiger counters and skipping the chemical precipitation, Curie and Savitch detected the 3.5-hour half-life in irradiated uranium.

With the Anschluss, Germany's unification with Austria on 12 March 1938, Meitner lost her Austrian citizenship. James Franck offered to sponsor her immigration to the United States, and Bohr offered a temporary place at his institute, but when she went to the Danish embassy for a visa, she was told that Denmark no longer recognised her Austrian passport as valid. On 13 July 1938, Meitner departed for the Netherlands with Dutch physicist Dirk Coster. Before she left, Otto Hahn gave her a diamond ring he had inherited from his mother to sell if necessary. She reached safety, but with only her summer clothes. Meitner later said that she left Germany forever with 10 marks in her purse. With the help of Coster and Adriaan Fokker, she flew to Copenhagen, where she was greeted by Frisch, and stayed with Niels and Margrethe Bohr at their holiday house in Tisvilde. On 1 August she took the train to Stockholm, where she was met by Eva von Bahr.

### Interpretation

The Paris group published their results in September 1938. Hahn dismissed the isotope with the 3.5-hour half-life as contamination, but after looking at the details of the Paris group's experiments and the decay curves, Strassmann was worried. He decided to repeat the experiment, using his more efficient method of separating radium. This time, they found what they thought was radium, which Hahn suggested resulted from two alpha decays:

238
92U + n → α + 235
90Th → α + 235
88Ra

Meitner found this very hard to believe.

In November, Hahn travelled to Copenhagen, where he met with Bohr and Meitner. They told him that they were very unhappy about the proposed radium isomers. On Meitner's instructions, Hahn and Strassmann began to redo the experiments, even as Fermi was collecting his Nobel Prize in Stockholm. Assisted by Clara Lieber and Irmgard Bohne, they isolated the three radium isotopes (verified by their half-lives) and used fractional crystallisation to separate them from the barium carrier by adding barium bromide crystals in four steps. Since radium precipitates preferentially in a solution of barium bromide, at each step the fraction drawn off would contain less radium than the one before. However, they found no difference between each of the fractions. In case their process was faulty in some way, they verified it with known isotopes of radium; the process was fine. On 19 December, Hahn wrote to Meitner, informing her that the radium isotopes behaved chemically like barium. Anxious to finish up before the Christmas break, Hahn and Strassmann submitted their findings to Naturwissenschaften on 22 December without waiting for Meitner to reply. Hahn understood that a "burst" of the atomic nuclei had occurred, but he was unsure about that interpretation. Hahn concluded the article in "Naturwissenschaften" with: "As chemists... we should substitute the symbols Ba, La, Ce for Ra, Ac, Th. As 'nuclear chemists' fairly close to physics we cannot yet bring ourselves to take this step which contradicts all previous experience in physics."

Frisch normally celebrated Christmas with Meitner in Berlin, but in 1938 she accepted an invitation from Eva von Bahr to spend it with her family at Kungälv, and Meitner asked Frisch to join her there. Meitner received the letter from Hahn describing his chemical proof that some of the product of the bombardment of uranium with neutrons was barium. Barium had an atomic mass 40% less than uranium, and no previously known methods of radioactive decay could account for such a large difference in the mass of the nucleus.

Nonetheless, she had immediately written back to Hahn to say: "At the moment the assumption of such a thoroughgoing breakup seems very difficult to me, but in nuclear physics we have experienced so many surprises, that one cannot unconditionally say: 'It is impossible.'" Meitner felt that Hahn was too careful a chemist to make an elementary blunder, but found the results difficult to explain. All the nuclear reactions that had been documented involved chipping protons or alpha particles from the nucleus. Breaking it up seemed far more difficult. However the liquid drop model that Gamow had postulated suggested the possibility that an atomic nucleus could become elongated and overcome the surface tension that held it together. According to Frisch:

> At that point we both sat down on a tree trunk (all that discussion had taken place while we walked through the wood in the snow, I with my skis on, Lise Meitner making good her claim that she could walk just as fast without), and started to calculate on scraps of paper. The charge of a uranium nucleus, we found, was indeed large enough to overcome the effect of the surface tension almost completely; so the uranium nucleus might indeed resemble a very wobbly unstable drop, ready to divide itself at the slightest provocation, such as the impact of a single neutron.But there was another problem. After separation, the two drops would be driven apart by their mutual electric repulsion and would acquire high speed and hence a very large energy, about 200 MeV in all; where could that energy come from? Fortunately Lise Meitner remembered the empirical formula for computing the masses of nuclei and worked out that the two nuclei formed by the division of a uranium nucleus together would be lighter than the original uranium nucleus by about one-fifth the mass of a proton. Now whenever mass disappears energy is created, according to Einstein's formula $E=m\,c^2$, and one-fifth of a proton mass was just equivalent to 200 MeV. So here was the source for that energy; it all fitted!

Meitner and Frisch had correctly interpreted Hahn's results to mean that the nucleus of uranium had split roughly in half. The first two reactions that the Berlin group had observed were light elements created by the breakup of uranium nuclei; the third, the 23-minute one, was a decay into the real element 93. On returning to Copenhagen, Frisch informed Bohr, who slapped his forehead and exclaimed "What idiots we have been!" Bohr promised not to say anything until they had a paper ready for publication. To speed the process, they decided to submit a one-page note to Nature. At this point, the only evidence that they had was the barium. Logically, if barium was formed, the other element must be krypton, although Hahn mistakenly believed that the atomic masses had to add up to 239 rather than the atomic numbers adding up to 92, and thought it was masurium (technetium), and so did not check for it:

235
92U + n →
56Ba +
36Kr + some n

Over a series of long-distance phone calls, Meitner and Frisch came up with a simple experiment to bolster their claim: to measure the recoil of the fission fragments, using a Geiger counter with the threshold set above that of the alpha particles. Frisch conducted the experiment on 13 January 1939, and found the pulses caused by the reaction just as they had predicted. He decided he needed a name for the newly discovered nuclear process. He spoke to William A. Arnold, an American biologist working with de Hevesy and asked him what biologists called the process by which living cells divided into two cells. Arnold told him that biologists called it fission. Frisch then applied that name to the nuclear process in his paper. Frisch mailed both the jointly-authored note on fission and his paper on the recoil experiment to Nature on 16 January 1939; the former appeared in print on 11 February and the latter on 18 February. In their second publication on nuclear fission in February 1939, Hahn and Strassmann used the term Uranspaltung (uranium fission) for the first time, and predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction.

## Reception

### Bohr brings the news to the United States

Before departing for the United States on 7 January 1939 with his son Erik to attend the Fifth Washington Conference on Theoretical Physics, Bohr promised Frisch that he would not mention fission until the papers appeared in print, but during the Atlantic crossing on the SS Drottningholm, Bohr discussed the mechanism of fission with Leon Rosenfeld, and failed to inform him that the information was confidential. On arrival in New York City on 16 January, they were met by Fermi and his wife Laura Capon, and by John Wheeler, who had been a fellow at Bohr's institute in 1934–1935. As it happened, there was a meeting of Princeton University's Physics Journal Club that evening, and when Wheeler asked Rosenfeld if he had any news to report, Rosenfeld told them. An embarrassed Bohr fired off a note to Nature defending Meitner and Frisch's claim to the priority of the discovery. Hahn was annoyed that while Bohr mentioned his and Strassmann's work in the note, he cited only Meitner and Frisch.

News spread quickly of the new discovery, which was correctly seen as an entirely novel physical effect with great scientific—and potentially practical—possibilities. Isidor Isaac Rabi and Willis Lamb, two Columbia University physicists working at Princeton, heard the news and carried it back to Columbia. Rabi said he told Fermi; Fermi gave credit to Lamb. For Fermi, the news came as a profound embarrassment, as the transuranic elements that he had partly been awarded the Nobel Prize for discovering had not been transuranic elements at all, but fission products. He added a footnote to this effect to his Nobel Prize acceptance speech. Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found Herbert L. Anderson. Bohr grabbed him by the shoulder and said: "Young man, let me explain to you about something new and exciting in physics."

### Further research

It was clear to many scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University group conducted the first nuclear fission experiment in the United States, which was done in the basement of Pupin Hall. The experiment involved placing uranium oxide inside of an ionization chamber and irradiating it with neutrons, and measuring the energy thus released. The next day, the Fifth Washington Conference on Theoretical Physics began in Washington, D.C., under the joint auspices of The George Washington University and the Carnegie Institution of Washington. From there, the news on nuclear fission spread even further, which fostered many more experimental demonstrations.

Bohr and Wheeler overhauled the liquid drop model to explain the mechanism of nuclear fission, with conspicuous success. Their paper appeared in Physical Review on 1 September 1939, the day Germany invaded Poland, starting World War II in Europe. As the experimental physicists studied fission, they uncovered more puzzling results. George Placzek asked Bohr why uranium fissioned with both very fast and very slow neutrons. Walking to a meeting with Wheeler, Bohr had an insight that the fission at low energies was due to the uranium-235 isotope, while at high energies it was mainly due to the far more abundant uranium-238 isotope. This was based on Meitner's 1937 measurements of the neutron capture cross-sections. This would be experimentally verified in February 1940, after Alfred Nier was able to produce sufficient pure uranium-235 for John R. Dunning, Aristid von Grosse and Eugene T. Booth to test.

Other scientists resumed the search for the elusive element 93, which seemed to be straightforward, as they now knew it resulted from the 23-minute half-life. At the Radiation Laboratory in Berkeley, California, Emilio Segrè and Edwin McMillan used the cyclotron to create the isotope. They then detected a beta activity with a 2-day half-life, but it had rare-earth element chemical characteristics, and element 93 was supposed to have chemistry akin to rhenium. It was therefore overlooked as just another fission product. Another year passed before McMillan and Philip Abelson determined that the 2-day half-life element was that of the elusive element 93, which they named "neptunium". They paved the way for the discovery by Glenn Seaborg, Emilio Segrè and Joseph W. Kennedy of element 94, which they named "plutonium" in 1941.

Another avenue of research, spearheaded by Meitner, was to determine if other elements could fission after being irradiated with neutrons. It was soon determined that thorium and protactinium could. Measurements were also made of the amount of energy released. Hans von Halban, Frédéric Joliot-Curie and Lew Kowarski demonstrated that uranium bombarded by neutrons emitted more neutrons than it absorbed, suggesting the possibility of a nuclear chain reaction. Fermi and Anderson did so too a few weeks later. It was apparent to many scientists that, in theory at least, an extremely powerful energy source could be created, although most still considered an atomic bomb an impossibility.

### Nobel Prize

Both Hahn and Meitner had been nominated for the chemistry and the physics Nobel Prizes many times even before the discovery of nuclear fission for their work on radioactive isotopes and protactinium. Several more nominations followed for the discovery of fission between 1940 and 1943. Nobel Prize nominations were vetted by committees of five, one for each award. Although both Hahn and Meitner received nominations for physics, radioactivity and radioactive elements had traditionally been seen as the domain of chemistry, and so the Nobel Committee for Chemistry evaluated the nominations in 1944.

The committee received reports from Theodor Svedberg in 1941 and Arne Westgren [sv] in 1942. These chemists were impressed by Hahn's work, but felt that the experimental work of Meitner and Frisch was not extraordinary. They did not understand why the physics community regarded their work as seminal. As for Strassmann, although his name was on the papers, there was a long-standing policy of conferring awards on the most senior scientist in a collaboration. In 1944 the Nobel Committee for Chemistry voted to recommend that Hahn alone be given the Nobel Prize in Chemistry for 1944. However, Germans had been forbidden to accept Nobel Prizes after the Nobel Peace Prize had been awarded to Carl von Ossietzky in 1936. The committee's recommendation was rejected by the Royal Swedish Academy of Sciences, which decided to defer the award for one year.

The war was over when the academy reconsidered the award in September 1945. The Nobel Committee for Chemistry had now become more cautious, as it was apparent that much research had been undertaken by the Manhattan Project in the United States in secret, and it suggested deferring the 1944 Nobel Prize in Chemistry for another year. The academy was swayed by Göran Liljestrand, who argued that it was important for the academy to assert its independence from the Allies of World War II, and award the Nobel Prize in Chemistry to a German, as it had done after World War I when it had awarded it to Fritz Haber. Hahn therefore became the sole recipient of the 1944 Nobel Prize in Chemistry "for his discovery of the fission of heavy nuclei".

Meitner wrote in a letter to her friend Birgit Broomé-Aminoff on 20 November 1945:

> Surely Hahn fully deserved the Nobel Prize in chemistry. There is really no doubt about it. But I believe that Otto Robert Frisch and I contributed something not insignificant to the clarification of the process of uranium fission – how it originates and that it produces so much energy, and that was something very remote from Hahn. For this reason I found it a bit unjust that in the newspapers I was called a Mitarbeiterin [subordinate] of Hahn's in the same sense that Strassmann was.

In 1946, the Nobel Committee for Physics considered nominations for Meitner and Frisch received from Max von Laue, Niels Bohr, Oskar Klein, Egil Hylleraas and James Franck. Reports were written for the committee by Erik Hulthén, who held the chair of experimental physics at Stockholm University, in 1945 and 1946. Hulthén argued that theoretical physics should be considered award-worthy only if it inspired great experiments. The role of Meitner and Frisch in being the first to understand and explain fission was not understood. There may also have been personal factors: the chairman of the committee, Manne Siegbahn, disliked Meitner, and had a professional rivalry with Klein. Meitner and Frisch would continue to be nominated regularly for many years, but would never be awarded a Nobel Prize.

## In history and memory

At the end of the war in Europe, Hahn was taken into custody and incarcerated at Farm Hall with nine other senior scientists, all of whom except Max von Laue had been involved with the German nuclear weapons program, and all except Hahn and Paul Harteck were physicists. It was here that they heard the news of the atomic bombings of Hiroshima and Nagasaki. Unwilling to accept that they were years behind the Americans, and unaware that their conversations were being recorded, many of them said in conversations, that they had never wanted their nuclear weapons program to succeed in the first place. Hahn did not believe them. Hahn was still there when his Nobel Prize was announced in November 1945. The Farm Hall scientists would spend the rest of their lives attempting to rehabilitate the image of German science that had been tarnished by the Nazi period. Inconvenient details like the thousands of female slave labourers from Sachsenhausen concentration camp who mined uranium ore for their experiments were swept under the rug.

For Hahn, this necessarily involved asserting his claim of the discovery of fission for himself, for chemistry, and for Germany. He used his Nobel Prize acceptance speech to assert this narrative, so he mentioned both Meitner's and Straßmann's involvements in his Nobel lecture. Hahn's message resonated strongly in Germany, where he was revered as the proverbial good German, a decent man who had been a staunch opponent of the Nazi regime, but had remained in Germany where he had pursued pure science. As president of the Max Planck Society from 1946 to 1960, he projected an image of German science as undiminished in brilliance and untainted by Nazism to an audience that wanted to believe it. After the Second World War, Hahn came out strongly against the use of nuclear energy for military purposes. He saw the application of his scientific discoveries to such ends as a misuse, or even a crime. Lawrence Badash wrote: "His wartime recognition of the perversion of science for the construction of weapons and his postwar activity in planning the direction of his country's scientific endeavours now inclined him increasingly toward being a spokesman for social responsibility."

In contrast, in the immediate aftermath of the war Meitner and Frisch were hailed as the discoverers of fission in English-speaking countries. Japan was seen as a puppet state of Germany and the destruction of Hiroshima and Nagasaki as poetic justice for the persecution of the Jewish people. In January 1946, Meitner toured the United States, where she gave lectures and received honorary degrees. She attended a cocktail party for Lieutenant General Leslie Groves, the director of the Manhattan Project (who gave her sole credit for the discovery of fission in his 1962 memoirs), and was named Woman of the Year by the Women's National Press Club. At the reception for this award, she sat next to the President of the United States, Harry S. Truman. But Meitner did not enjoy public speaking, especially in English, nor did she relish the role of a celebrity, and she declined the offer of a visiting professorship at Wellesley College. Hahn nominated Meitner and Frisch for the Nobel Prize in Physics in 1948. He and Meitner remained close friends after the war.

In 1966, the United States Atomic Energy Commission jointly awarded the Enrico Fermi Award to Hahn, Strassmann and Meitner for their discovery of fission. The ceremony was held in the Hofburg palace in Vienna. It was the first time that the Enrico Fermi Prize had been awarded to non-Americans, and the first time it was presented to a woman. Meitner's diploma bore the words: "For pioneering research in the naturally occurring radioactivities and extensive experimental studies leading to the discovery of fission". Hahn's diploma was slightly different: "For pioneering research in the naturally occurring radioactivities and extensive experimental studies culminating in the discovery of fission." Hahn and Strassmann were present, but Meitner was too ill to attend, so Frisch accepted the award on her behalf.

During combined celebrations in Germany of the 100th birthdays of Einstein, Hahn, Meitner and von Laue in 1978, Hahn's narrative of the discovery of fission began to crumble. Hahn and Meitner had died in 1968, but Strassmann was still alive, and he asserted the importance of his analytical chemistry and Meitner's physics in the discovery, and their role as more than just assistants. A detailed biography of Strassmann appeared in 1981, a year after his death, and a prize-winning one of Meitner for young adults in 1986. Scientists questioned the focus on chemistry, historians challenged the accepted narrative of the Nazi period, and feminists saw Meitner as yet another example of the Matilda effect, where a woman had been airbrushed from the pages of history. By 1990, Meitner had been restored to the narrative, although her role remained contested, particularly in Germany.

Weizsäcker, a colleague of Hahn and Meitner during their time in Berlin, and a fellow inmate with Hahn in Farm Hall, strongly supported Hahn's role in the discovery of nuclear fission. He told an audience that had gathered for the ceremonial inclusion of a bust of Meitner in the Ehrensaal (Hall of Fame) at the Deutsches Museum in Munich on 4 July 1991 that neither Meitner nor physics had contributed to the discovery of fission, which, he declared, was "a discovery of Hahn's and not of Lise Meitner's."

1938 achievement in physics