Stalin pursued a strategy of facilitating the formation of a Polish government independent of ( and in opposition to ) the exile government in London by empowering the Polish communists . Among Polish communist organizations established during the war were the Polish Workers ' Party in occupied Poland and the Union of Polish Patriots in Moscow . A new Polish army was formed in the Soviet Union to fight together with the Soviets . At the same time Stalin worked on co-opting the Western Allies ( the United States led by President Franklin D. Roosevelt and the United Kingdom led by Prime Minister Winston Churchill ) , who , in terms of practical implementations , conformed to Stalin 's views on Poland 's borders and future government . The fate of Poland had been determined in a series of negotiations that included the conferences in Tehran , Yalta , and Potsdam . In 1944 , the Polish Government-in-Exile approved and the underground in Poland undertook unilateral political and military actions aimed at establishing an independent Polish authority , but the efforts were thwarted by the Soviets . The Polish communists founded the State National Council in 1943/44 in occupied Warsaw and the Polish Committee of National Liberation in July 1944 in Lublin , after the arrival of the Soviet army . The Soviet Union kept the eastern half of prewar Poland , granting Poland instead the greater southern portion of the eliminated German East Prussia and shifting the country west to the Oder – Neisse line , at the expense of Germany
Yalta
Yalta Conference
History of Poland (1939–1945)