As agreed by the Allies in Yalta , the Soviet Union incorporated the lands in eastern Poland ( Kresy , east of the Curzon Line ) , previously occupied and annexed in 1939 ( see Territories of Poland annexed by the Soviet Union ) . Deferring to Stalin 's territorial schemes , the Allies compensated Poland with the German territories east of the Oder – Neisse line , parts of Pomerania , Silesia and East Prussia ( in Polish communist government 's propaganda referred to as the Recovered Territories ) . The deal was practically , but in principle not permanently , finalized at the Potsdam Conference ( 17 July to 2 August 1945 ) . The entire country was shifted to the west and resembled the territory of Medieval early Piast Poland . Per the Potsdam agreement , millions of Germans were expelled and forced to relocate their families to the new Germany . About 4.4 million had already fled not waiting for the Potsdam decrees ( most during the final months of the war ) , and 3.5 million were removed from what was now territory of Poland in 1945 – 1949 . Davies wrote that the resettlement of Germans was not merely an act of wartime revenge , but a result of decades old Allied policy . The Russians as well as the British saw the German East Prussia as a product of German militarism , the " root of Europe 's miseries " , and the Allies therefore intended to eradicate it
Potsdam Conference
Potsdam Conference
History of Poland (1939–1945)