In September , 1939 the Soviet Union occupied the eastern voivodeships of Poland . During World War II , the occupied territory was annexed and ceded to the Soviet republic of : Ukrainian SSR , Byelorussian SSR and the Lithuanian Soviet Socialist Republic . The annexation of this part of Poland was accepted at the Tehran and Yalta Conference . As a result , under the pressure of world powers , Poland was forced to cede the Soviet Union 48 % of its territory , equating to the loss of 178,000 km ² of land , gaining the equivalent of 101 000 km ² in the west . The modification of Poland 's borders by the United Kingdom and the United States of America was a direct violation of the Atlantic Charter
Yalta Conference
Yalta Conference
Polish material losses during World War II