Before the war, the Soviets and the Nazis had a pact,
Molotov-Ribbentrop Pact, which allowed them to split up major portions of europe..." with Ribbentrop insisting, "there was no problem between the Baltic and the
Black Sea that could not be solved between the two of us."
[9][19][20] German officials stated that, unlike Britain, Germany could permit the Soviets to continue their developments unmolested, and that "there is one common element in the ideology of Germany, Italy and the Soviet Union: opposition to the capitalist democracies of the West"
https://en.wikipedia.org/wiki/Soviet_Union_in_World_War_II
This pact endured into ww2, and became untethered due to the fog of war? or Hitler or Stalin having changing their mind or having false intentions when signing the pact. It is unclear.
But disagreements over the splitting of Poland and other countries led to the pact's demise.
What we do know is that the communists began the war on the side of the Nazis. And there is no evidence whatsoever that the commies changed sides because they 'didn't like the extermination of Jews. None'
It appears that it was a matter of disagreement over the splitting up of countries.
This entire debate is currently because the left wants to justify their support of commie antifa because 'commies fight nazis'.
But commie antifa has no more intention of protecting Jews than obongo had of stating the phrase' radical islam'.
Antifa wants to run an authoritarian dictatorship in the US where they set the rules of who can talk and who can't and it won't stop there.