The Soviet Union immediately breaks the Molotov-Ribbentrop Pact in October 1939, and continues marching west, invading western Poland and Germany. The reason might be a secret pact with the Allies, or maybe Stalin isn't in charge and someone with ideals more akin to Trotsky's is. Whatever the reason, it happens.
Do the Western Allies attempt an invasion of Germany from the west? Do the Germans push the Soviets back to Moscow?