The Red Army , which vastly outnumbered the Polish defenders , achieved its targets encountering only limited resistance . Some 320,000 Polish prisoners of war had been captured . The campaign of mass persecution in the newly acquired areas began immediately . In November 1939 the Soviet government ostensibly annexed the entire Polish territory under its control . Some 13.5 million Polish citizens who fell under the military occupation were made into new Soviet subjects following show elections conducted by the NKVD secret police in the atmosphere of terror , the results of which were used to legitimize the use of force . A Soviet campaign of political murders and other forms of repression , targeting Polish figures of authority such as military officers , police and priests , began with a wave of arrests and summary executions . The Soviet NKVD sent hundreds of thousands of people from eastern Poland to Siberia and other remote parts of the Soviet Union in four major waves of deportation between 1939 and 1941 . Soviet forces occupied eastern Poland until the summer of 1941 , when they were driven out by the German army in the course of Operation Barbarossa . The area was under German occupation until the Red Army reconquered it in the summer of 1944 . An agreement at the Yalta Conference permitted the Soviet Union to annex almost all of their Molotov – Ribbentrop Pact portion of the Second Polish Republic , compensating the People 's Republic of Poland with the southern half of East Prussia and territories east of the Oder – Neisse line . The Soviet Union enclosed most of the conquered annexed territories into the Ukrainian Soviet Socialist Republic and the Byelorussian Soviet Socialist Republic
Yalta Conference
Yalta Conference
Soviet invasion of Poland