With a P.O.D. of September 1, 1939, your challenge is to keep the states of central and eastern Europe from becoming outright Soviet puppets at the end of WWII. They can be within the Soviet sphere on influence, but need to maintain a good deal of autonomy (no rigged elections and whatnot). You can still have socialist states in Eastern Europe, but only if they are fairly elected, and remain independent of Soviet socialism (No Red Army tanks rolling in if the socialists are voted out).