The Nazis were allies with the Communists in 1939. Germany and Soviet Russia dismembered Poland as allies, as a team. Molotov-Ribbentrop? How did the Katyn forest massacre occur if the Russians weren’t part of the invasion of Poland in 1939?
Temporary allies, an expedient. Fundamental enemies. Both knew there was a deathmatch coming.
And all of Eastern Europe was caught between them.