Part of the Paraguayan War
The Passage of Humaitá (Portuguese: "Passagem de Humaitá") was an operation of riverine warfare during the Paraguayan War − the most lethal in South American history − in which a force of six Imperial Brazilian Navy armoured vessels was ordered to dash past under the guns of the Paraguayan fortress of Humaitá. Some competent neutral observers had considered that the feat was very nearly impossible.
The purpose of the exercise was to stop the Paraguayans resupplying the fortress by river, and to provide the Empire of Brazil and its Allies with a much-needed propaganda victory. The attempt took place on 19 February 1868 and was successful – the attackers had hit upon the fortress' Achilles heel. It restored the reputation of the Brazilian navy and the Brazilian Empire's financial credit, and caused the Paraguayans to evacuate their capital Asunción. Some authors have considered that it was the turning point or culminating event of the war. The fortress, by then fully surrounded by Allied forces on land or blockaded by water, was captured on 25 July 1868.
Historical background.
The Fortress of Humaitá was a defensive system on a bend in the River Paraguay, not far from its mouth. Long considered to be almost impregnable, it prevented enemy shipping ascending the river and hence invading the Republic of Paraguay. There may have been other routes by which the country could have been invaded, but they were considered more difficult still. From the main article on the Fortress of Humaitá it appears that Marshal-President López of Paraguay may have calculated that, because the fortress could not be bypassed except at ruinous expense, he could risk making war on the much more populous Brazil and Argentina.
The Paraguayan War (also known as the War of the Triple Alliance) of 1864–1870 was the most lethal in South American history, and − in terms of its relative mortality − very possibly the worst in modern history, anywhere. It actually commenced when López seized the Brazilian ship "Marques de Olinda" on her routine voyage up the River Paraguay to the Brazilian province of Mato Grosso, then sent military forces to invade the province itself. It developed further when he seized Argentine naval vessels moored in the port of Corrientes, north east Argentina. Thereafter López sent two further armies, one to invade the Argentine province of Corrientes along the River Paraná and the other to invade the Brazilian province of Rio Grande do Sul along the River Uruguay. On 1 May 1865 Brazil, Argentina and Uruguay signed the Treaty of the Triple Alliance by which they would not negotiate peace with Paraguay until the government of López had been deposed.
Further, for Brazil the river was the only practicable route to its interior province of Mato Grosso, which had been invaded by the forces of López earlier in the War. The war aims of the Allies (Brazil, Argentina and Uruguay) explicitly included the capture and destruction of Humaitá, and it was the prime military objective of the Allied high command.
The Allies' strategic problem.
It took the Allies about a year to defeat the invading Paraguayan forces in the Brazilian province of Rio Grande do Sul and expel them from Argentina. According to their treaty they now had to neutralise Humaitá and depose López. In April 1866 the Allies crossed the River Paraná into southwestern Paraguay, occupying a small strip of territory. Their next strategic problem was how to proceed up the River Paraguay, getting past the Fortress of Humaitá.
Briefly, the fortress of Humaitá was built on a sharp concave bend in the river and comprised more than a mile of heavy artillery batteries atop a low cliff. The channel was only 200 yards wide, and ran within easy range of the batteries; a heavy chain boom could be raised to block the navigation and detain the shipping under the guns. 'Torpedoes' (improvised contact naval mines) could be released or anchored in the stream. On its landward side the fortress was protected either by impassable terrain or by 8 miles of trenches with 120 heavy guns and a garrison of 18,000 men. An attempt to capture one of its outworks by frontal attack failed disastrously at the Battle of Curupayty (22 September 1866); another frontal attack was out of the question.
From a naval perspective the heavy guns of the fortress might – in principle – be bypassed by the latest ironclads (armoured vessels), provided the chain boom could be cut and any 'torpedoes' avoided. But those vessels could not have operated ahead of their forward bases for long, because they would need to be resupplied with fuel, ammunition and provisions. Furthermore, the Brazilian navy command claimed there was no room inside the ironclads for the conveyance of troops; the wooden transport vessels could not have accompanied them.
The land operation in which the Allied land forces did succeed in capturing the Humaitá complex, known as the Siege of Humaitá, is outside the scope of this Article. However, to starve out Humaitá it was necessary to stop the Paraguayans resupplying it by river. But, in order to do that, it was necessary to obtain command of the river, in other words, to steam past Humaitá.
Difficulties facing the Allies.
1. The home front.
By the end of 1867 the Allies were badly in need of a propaganda victory. The war had cost them a lot of men and money; though they still had superior manpower yet they were stuck in the small area of southwestern Paraguay shown in the map. The disaster of Curupayty had sunk their morale and led to a year's inactivity. And it had fostered an already strong anti-war faction in Argentina. Armed revolutions had broken out in that country – especially in the Andean provinces – demanding peace with Paraguay. It had got to the point that the Allied commander-in-chief General Bartolomé Mitre had temporarily quit Paraguay with 4,000 Argentine troops in order to put down the revolts, leaving the Brazilian General the Marquis of Caxias in charge.
Brazil financed its warlike operations partly by borrowing in the financial markets especially the City of London, and advanced money to Argentina and Uruguay. The lack of progress in the war was affecting its financial credit.
In June 1867 Caxias wrote to a colleague: 
My friend, let us see if we can conclude this cursed war which has ruined our country, and whose duration shames us.
2. Extended lines of communication.
Lengthy lines of communication vastly increased the difficulty and cost of the war for Brazil. According to the American envoy to Paraguay, López had banked on that fact when deciding to make war on the Brazilian Empire in the first place.
Steam vessels were the only practicable means of communication. Thus in Brazil the overland journey from São Paulo to Mato Grosso took two and a half months by fast mule; by steamship, although the detour south through the River Plate and up through the River Paraguay was vast, it could be done in little over a fortnight. Tension over this route had been one of the causes of the Paraguayan war.
Ammunition or other stores shipped from Rio de Janeiro, Brazil to the naval arsenal at Isla del Cerrito, near the mouth of the river Paraguay, had to travel an extensive water route.
Vessels requiring to be repaired could not be sent down to Buenos Aires for lack of facilities, and Rio de Janeiro was too far. Isla del Cerrito ("island of the hillock") was and is a small island at the confluence of the Paraná and Paraguay rivers. There the navy established a machine shop employing 20 men; a boatyard manned by 50 carpenters, etc; a coal bunker; a gunpowder mill; a naval hospital; and even a church. Good naval timber required for repairs was harvested locally.
Because the river was blocked at Humaitá, stores required further north had to go by oxcart convoys on a route flanking Humaitá to the east. Those convoys were the object of repeated Paraguayan ambushes.
3. Inherent difficulty in effecting the naval passage of Humaitá.
Although some foreign observers were critical of the Brazilian navy's lack of zeal and implied that it should have forced the passage long ago, those observers were not naval men. One who was, Commander Kennedy of the Royal Navy, wrote that there were real difficulties, which he identified as these:
Commander Kennedy's evaluation:
It is difficult to conceive a more formidable obstacle to an advancing squadron than this small portion of the river between Tres Bocas and Humaitá.
Further, although ironclad vessels were not supposed to be sunk by river batteries of the type possessed by Paraguay, they could be badly damaged. Cannonballs sometimes did penetrate the hull by going through a porthole or gunport. When this happened the shot could fracture or ricochet around inside the casemate producing appalling carnage. And even though most shots did not get inside the hull their force could start the bolts of the iron plating, or produce combat wounds by splintering the wooden backing. For inexperienced seamen inside these vessels the effect could be terrifying.
In any case it would have been necessary, somehow, to cut or evade the chain boom, or else be detained under the Paraguayan artillery indefinitely.
According to the English-language "Buenos Ayres Standard" Experienced American and English and French naval officers, who had seen Humaitá, inspected the position, and gone through the batteries, all unanimously agreed on its extreme strength. 
4. Dissension amongst the commands.
The capture of Humaitá was an early example in modern warfare of combined operations, requiring cooperation between land and naval forces but as will now be described there were serious problems.
Argentina and Brazil were traditional enemies. The alliance between them was that of "dog and cat", only brought about when López declared war on them both; it was fraught with mistrust and veiled antagonism. For political reasons the Argentine General Bartolomé Mitre was nominally commander-in-chief, but this went down badly in Brazil whose land forces were much larger; in reality Mitre had to seek consensus with the Brazilian and Uruguayan commands.
Further, the Treaty of the Triple Alliance had stipulated that the naval forces (to all intents and purposes the Brazilian navy, for the Argentine naval forces were small) were to be "under the immediate command" of the Brazilian admiral; and the Brazilian government made it plain that the navy was not taking orders from Mitre. 
During the war the Brazilian navy had been criticised for its inertia. It was the modern ironclad vessels of the Brazilian navy that had stood by and allowed the retiring Paraguayan army to escape across the River Paraná on rafts – together with 100,000 head of rustled cattle – without doing anything to stop them. Those same vessels had been defied by two Paraguayan "chatas" (flat-bottomed, towable barges mounting a single gun). Although the Brazilian naval command protested that the shallow river waters enjoined caution, the Brazilian soldiery and the Emperor Pedro II were disgusted. As a result, the Brazilian Admiral Tamandaré had been relieved of his command and Joaquim José Inácio (1808–1869) had taken over his functions on 2 December 1866.
The naval command was thoroughly distrusted by General Mitre and the distrust was mutual, for the Brazilian commands, military and naval, suspected the Argentines would be only too pleased if the Brazilian navy was destroyed in the war, since it would weaken Brazilian power in the River Plate region. The more Mitre asked the navy to be bold, the more they suspected him.
In practice Mitre never attempted to give direct orders to the Brazilian navy, instead seeking to persuade it through the senior Brazilian land commander.
5. Ageing naval command.
The new admiral Inácio had had a distinguished and brave fighting career in the days of the blue-water sailing navy. But he had not been to sea for years, pursuing a career as a bureaucrat and then a politician. In Paraguay Inácio became unwell, a prematurely old man, "a ghost of an admiral", as the modern Brazilian historian Francisco Doratioto put it.
According to Artur Silveira da Mota, commander of the ironclad "Barroso" (who was a full naval captain by the age of 26, and later in life given the rank of admiral and made Baron of Jaceguai) "most of the senior naval command showed themselves to be incompetent for war service, whether for advancing years or for being too long in sedentary jobs",