Referendum on whether to become part of Poland
The East Prussian plebiscite (), also known as the Allenstein and Marienwerder plebiscite or Warmia, Masuria and Powiśle plebiscite (), was a plebiscite for the self-determination of the regions of southern Warmia (Ermland), Masuria (Mazury, Masuren) and Powiśle, which had been in parts of the East Prussian Government Region of Allenstein and of the West Prussian Government Region of Marienwerder in accordance with Articles 94 to 97 of the Treaty of Versailles.
Prepared in early 1920, the plebiscite took place on 11 July 1920 and was conducted by German authorities under Inter-Allied control.
The plebiscite reported that most voters had selected East Prussia over Poland (over 97% in the Allenstein Plebiscite Area and 92% in the Marienwerder Plebiscite Area). As a result, most of the territories in question remained in the Free State of Prussia, and therefore, in Germany.
Background.
The area concerned had changed hands at various times over the centuries between the Old Prussians, the monastic state of the Teutonic Knights, the Duchy of Prussia, Germany and Poland. The area of Warmia had been part of the Kingdom of Prussia since the first Partition of Poland in 1772, and the region of Masuria was ruled by the German Hohenzollern family since the Prussian Tribute of 1525, as a Polish fief until 1660. Many inhabitants of that region had Polish roots and were influenced by Polish culture. The last official German census in 1910 classified them as ethnic Poles or Masurians.
During the German Empire, harsh Germanisation measures were enacted in the region. The Polish delegation at the Paris Peace Conference, led by Roman Dmowski, made a number of demands in relation to areas that had been part of the Polish–Lithuanian Commonwealth until 1772. Despite their protests (supported by the French), US President Woodrow Wilson and the other Allies agreed that plebiscites according to self-determination should be held.
In the former German Province of Posen and parts of West Prussia, an armed revolt had already removed the German authorities in 1919.
Areas.
 
The "plebiscite areas" (; ) were placed under the authority of two Inter-Allied Commissions of five members, who were appointed by the Principal Allied and Associated Powers representing the League of Nations. British and Italian troops, under the command of the Commissions, arrived on and soon after 12 February 1920 after the regular German "Reichswehr" had previously left the plebiscite areas. The civil and municipal administration was continued by the existing German authorities, which were responsible to the Commissions for their duration.
In accordance with Articles 94 to 97 of the Treaty of Versailles (section entitled "East Prussia"), the Marienwerder Plebiscite Area was formed of northeastern Marienwerder Government Region, based in Marienwerder in West Prussia, now Kwidzyn, which encompassed the districts of Marienwerder (east of the Vistula), Stuhm (based in Stuhm, now Sztum), Rosenberg (based in Rosenberg in West Prussia, now Susz) as well as parts of Marienburg in West Prussia (based in Marienburg in West Prussia, Malbork, part of the Danzig Government Region) east of the Nogat. The treaty defined the Allenstein Plebiscite Area as "The western and northern boundary of Allenstein Government Region to its junction with the boundary between the districts of Oletzko (based in Marggrabowa, now Olecko) and of Angerburg (based in Angerburg, now Węgorzewo). The northern boundary of the Oletzko District to its junction with the old frontier of East Prussia." Thus, the Allenstein precinct comprised all the Allenstein Region plus the Oletzko District (Gumbinnen Government Region).
According to Jerzy Minakowski, the area of the plebiscite had 720,000 people, who were German citizens, and 440,000 of them were considered Polish by him by their Mazurian dialect of Polish. The official Prussian census of 1910 showed 245,000 Polish- and Mazurian-speakers and 289,000 German-speakers in the Allenstein Government Region and 23,000 and 136,000 in the Marienwerder Government Region. 
Allenstein / Olsztyn Plebiscite Area.
The Allied forces had to intervene here in 1919 to release imprisoned Masurians, who had tried to reach the Paris Conference. 
The president and British commissioner of the Inter-Allied Administrative and Plebiscite Commission for Allenstein was Ernest Rennie; the French commissioner was Couget; the Italian commissioner was Marquis Fracassi, a senator; and the Japanese commissioner was Marumo. The German government, under the Protocol's terms, was allowed to attach a delegate and sent "Reichskommissar" Wilhelm von Gayl, who had been in the service of the Interior Ministry before he was on the Inner Colonisation Committee. The local police forces were placed under the control of two British officers: Lieutenant-Colonel Bennet and Major David Deevis. Bennet reported that he regarded them as "well-disciplined and reliable". There was also a battalion from the Royal Irish Regiment and an Italian regiment stationed at Lyck (Ełk).
According to Jerzy Minakowski, the small forces had proven themselves inadequate to protect pro-Polish voters in the precincts from pro-German repressions. 
The Commission had general powers of administration and was particularly "charged with the duty of arranging for the vote and of taking such measures as it may deem necessary to ensure its freedom, fairness, and secrecy. The Commission will have all necessary authority to decide any questions to which the execution of these provisions may give rise. The Commission will make such arrangements as may be necessary for assistance in the exercise of its functions by officials chosen by itself from the local population. Its decisions will be taken by a majority".
The commission was welcomed by the Poles in the region, who hoped that its presence would improve their situation, but petitions were made to remove German officials and the "Sicherheitswehr" and demanded for the official welcoming committee of German officials to show the representatives of the Allies the plight of ethnic Poles.
On 18 February 1919, the Allenstein-based commission decreed that the Polish language would gain equal rights to the German language in the region.
The commission eventually had to remove both the mayor of Allenstein, Georg Zülch, and an officer of "Sicherheitswehr", Major Oldenburg, after a Polish banner at the local consulate of Poland was defaced. The Poles expressed gratitude for Allied protection of Polish rights and underlined their desire for peaceful co-existence with the Germans.
In April 1920, during a Polish theatrical performance in Deuthen (Dajtki), near Allenstein, Poles were attacked by pro-German activists. As demanded by the Allied Commission, the German police escorted Polish actors, but they then ignored the attackers. In Bischofsburg (Biskupiec), a pogrom against Poles was organised, which prompted the creation of a special commission to find the perpetrators. The "Allensteiner Zeitung" newspaper called on its readers to remain calm and to cease pogroms against Poles and pointing out that they could lead to postponing the plebiscite, which would go against German interests.
Italian forces were sent to Lötzen (Giżycko), according to Jerzy Minakowski, to protect the Poles after a pogrom had occurred on 17 April. In May, several attacks on Poles were reported in Osterode (Ostróda) and included attacks on coworkers of the Masurian Committee.
Marienwerder / Kwidzyn Plebiscite Area.
Parts of the Marienwerder Government Region were confined as the Marienwerder Plebiscite Area. The commission for the plebiscite area reached Marienwerder (Kwidzyn) on 17 February 1920. Upon its arrival, it found an Italian battalion of "Bersaglieri" on guard that afterwards marched past at the double. The commission had about 1,400 uniformed German police under its authority. Beaumont was accused by Poles of having a cold and ironic attitude toward them.
Beaumont said that with the exception of the Kreis Stuhm, where Poles admittedly numbered 15,500 out of a population of 36,500 (42%), they had Polish sympathies since they were Catholics. In the other districts, with the exception of Allenstein, Poles depicting themselves as Mazurians were Lutherans and German in a national conviction.
On the eve of the plebiscite, Beaumont reported Poles strictly guarding the new frontier between East Prussia and Poland to prevent people from passing to East Prussia without vexatious formalities. They held up trains for hours and constantly interrupted or even completely suppressed postal, telegraphic and telephonic communication service. The great bridge over the Vistula at Dirschau was barred by sentries, in French uniforms, "who refuse to understand any language but Polish". As a result, Beaumont wrote that area to have been "cut off from its shopping centre and chief port almost completely".
After the plebiscite, the bridge was removed. To Beaumont it would be "desirable to convey a hint to the Warsaw Government that their present policy is scarcely calculated to gain them votes".
Sir Horace Rumbold, the British minister in Warsaw, also wrote to Curzon on 5 March 1920 that the Plebiscite Commissions at Allenstein and Marienwerder "felt that they were isolated both from Poland and from Germany" and that the Polish authorities were holding up supplies of coal and petrol to those districts. Rumbold had a meeting with the Polish Minister for Foreign Affairs, Stanisław Patek, who declared that he was disappointed with his people's behaviour and "spoke strongly about the tactlessness and rigidity of the Polish Military authorities".
On 10 March 1920 Beaumont wrote of numerous continuing difficulties being made by Polish officials and stressed the "ill-will between Polish and German nationalities and the irritation due to Polish intolerance towards the German inhabitants in the Corridor (now under their rule), far worse than any former German intolerance of the Poles, are growing to such an extent that it is impossible to believe the present settlement (borders) can have any chance of being permanent...".
The Poles began to harden their position, and Rumbold reported to Curzon on 22 March 1920 that Count Stefan Przeździecki, an official of the Polish Foreign Office, had told Sir Percy Loraine (first secretary at the legation at Warsaw) that the Poles questioned the impartiality of the Inter-Allied Commissions and indicated that the Polish government might refuse to recognise the results of the plebiscites.
Infiltration attempts of Polish irregulars into the Marienwerder area were checked by Italian troops.
Propaganda.
German "Heimatdienst".
Both sides started a propaganda campaign. In March 1919 Paul Hensel, the Lutheran Superintendent of Johannisburg, had travelled to Versailles to hand over a collection of 144,447 signatures to the Allies to protest the planned cession. Pro-German campaigners collected several regional associations under the "Ostdeutscher Heimatdienst" (East German Homeland Service), which collected over 220,000 members. The "Heimatdienst" in the region was led by Max Worgitzki, an author and publisher of the "Ostdeutsche Nachrichten".
The "Heimatdienst" exerted strong psychological pressure on Masurians to vote for Germany and threatened Polish forces with physical violence. They appealed to Prussian history and loyalty to the Prussian state, disqualified Polish culture and warned of the Catholic religion and Poland's alleged economical backwardness. The pro-Germans presented the probability that all men would be drafted into the Polish military to fight Soviet Russia if they would vote for the annexation by Poland. A Soviet invasion was then being attempted in the alleged eastern parts of Poland.
Those were no new standpoints but had been the prevailing common sense in Mazurian public mind for decades. The German nationalist feelings were recently strengthened even more by the massive rebuilding programme of the devastated towns, which had been destroyed during the Russian invasion in the autumn of 1914 and were then financially adopted by large German cities. Rennie, the British commissioner in Allenstein, reported on 11 March 1920, that "in those parts which touch the Polish frontier a vigorous German propaganda is in progress" and that "the Commission is doing all it can to prevent German officials in the district from taking part in national propaganda in connection with the Plebiscite. Ordinances and instructions in this sense have been issued".
Polish campaign.
A delegation of Masurians petitioned the Allies in March 1919 to join their region with Poland.
The Poles established an unofficial Masurian Plebiscite Committee ("Mazurski Komitet Plebiscytowy") on 6 June 1919 that was chaired by a Polish citizen Juliusz Bursche, later Bishop of the Evangelical-Augsburg Church in Poland. There was also an unofficial Warmian Plebiscite Committee. They tried to convince the Masurians of Warmia (Ermland) and Masuria that they were victims of a long period of Germanisation but that Poles now had the opportunity to liberate themselves from Prussian rule.
Rennie reported to Curzon at the British Foreign Office on 18 February 1920, that the Poles, who had taken control of the Polish Corridor to the Baltic Sea, had "entirely disrupted the railway, telegraphic and telephone system, and the greatest difficulty is being experienced".
Rennie reported on 11 March 1920 the arrival of the