Same thing happened in WWI. Besides, Russia brought the invasion on itself via the Molotov-Ribbentrop Pact, which opened the way for the German invasion of the West. The Russians also provided the Germans with the war materials with which they attacked France and the Low Countries. Once Western Europe was overrun, the Germans could invade Russia without worrying about the French reaction.
Okay, so we’ve beat the first two world wars to death. How about we not go poking a bear with a history sacrificing tens of millions of its own people to level the cities of an invading power?