Posen-West Prussia in the context of "Province of Posen"

The entirety of Germany was occupied and administered by the Allies of World War II, from the Berlin Declaration on 5 June 1945 to the establishment of West Germany on 23 May 1949. Unlike occupied Japan, Nazi Germany was stripped of its sovereignty and its government was entirely dissolved. After Germany formally surrendered on Tuesday, 8 May 1945, the four countries representing the Allies (the United States, United Kingdom, Soviet Union, and France) asserted joint authority and sovereignty through the Allied Control Council (ACC).

Germany after the war was a devastated country – roughly 80 percent of its infrastructure was in need of repair or reconstruction – which helped the idea that Germany was entering a new phase of history ("zero hour"). At first, Allied-occupied Germany was defined as all territories of Germany before the 1938 Nazi annexation of Austria. The Potsdam Agreement on 2 August 1945 defined the new eastern German border by giving Poland and the Soviet Union all regions of Germany east of the Oder–Neisse line (eastern parts of Pomerania, Neumark, Posen-West Prussia, East-Prussia and most of Silesia) and divided the remaining "Germany as a whole" into four occupation zones, each administered by one of the Allies.

The Province of West Prussia (German: Provinz Westpreußen; Kashubian: Zôpadné Prësë; Polish: Prusy Zachodnie) was a province of Prussia from 1773 to 1829 and from 1878 to 1919. West Prussia was established as a province of the Kingdom of Prussia in 1773, formed from Royal Prussia of the Polish–Lithuanian Commonwealth annexed in the First Partition of Poland. West Prussia was dissolved in 1829 and merged with East Prussia to form the Province of Prussia, but was re-established in 1878 when the merger was reversed and became part of the German Empire. From 1918, West Prussia was a province of the Free State of Prussia within Weimar Germany, losing most of its territory to the Second Polish Republic and the Free City of Danzig in the Treaty of Versailles. West Prussia was dissolved in 1919, and its remaining western territory was merged with Posen to form Posen-West Prussia, and its eastern territory merged with East Prussia as the Region of West Prussia district.

West Prussia's provincial capital alternated between Marienwerder (present-day Kwidzyn, Poland) and Danzig (Gdańsk, Poland) during its early existence. West Prussia was notable for its ethnic and religious diversity due to immigration and cultural changes, with the population becoming mixed over the centuries. Since the early Middle Ages the bulk of the region was inhabited by West Slavic Lechitic tribes (Pomeranians in the Pomerelia region and Masovians in Kulmerland), while the actual Old Prussians (Pomesanians and Pogesanians) populated only the remaining part of the territory lying to the east of the Vistula River. The Teutonic Order's conquest of the region resulted in German colonization in the 14th century. As a result of Germanisation, Germans became in the middle of the 19th century the most numerous ethnic group in West Prussia as a whole, remaining as such until the dissolution of the province in 1920, though their distribution was uneven: their majority was concentrated in Danzig, the western lands of the province, along the Vistula river, and in the Pomesanian and Pogesanian portion of the province located east of the Vistula, with a small admixture of Poles (Gedanians and Powiślans). Meanwhile, Poles (Kociewians, Borowians and Chełminians) as well as Kashubians continued to predominate in parts of Pomerelian territories west of Vistula and in parts of the Chełmno Land, forming altogether around 36% of the population of the province as a whole. There were also sizeable minorities of Mennonites and Jews settling in the region.