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
Russia involvement in regime change