Soviet invasion of Poland
Soviet forces stayed in eastern Poland until the summer of 1941, when the German Army invaded Operation Barbarossa. The area was under Nazi occupation until the Red Army reconquered it in the summer of 1944. An agreement at the Yalta Conference let the Soviet Union keep almost all of what it had occupied in the Second Polish Republic.  The People's Republic of Poland got the southern half of East Prussia and lands east of the Oder-Neisse Line. The Soviet Union put those areas into the Ukrainian Soviet Socialist Republic and the Byelorussian Soviet Socialist Republic.