Authoritarian leftism defeated Naziism because the Nazis gave them no choice--Stalin was perfectly happy to cooperate with Hitler to carve up Poland between the two of them (c.f. Molotov-Ribbentrop Pact). But Hitler hated communists almost as much as he hated Jews (and often conflated the two), so betrayed Stalin at what seemed like an opportune moment.

Prior to that, nobody was a more gleeful partner to Nazi aggression than the prototypical authoritarian leftist Stalin.

And if Russia hadn't fought back the Nazis, they would have lost anyway, just a little bit later.

Not sure whether this is propaganda or just you being misinformed, but whatever it is, it's very wrong.