Soviet forces stayed in eastern Poland until the summer of 1941. They were moved by the invading German army in the course of 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 add almost all of their part of 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 added the lands into the Ukrainian Soviet Socialist Republic and the Byelorussian Soviet Socialist Republic.