If the Russians hadn’t signed the 1939 Non-Aggression Pact with Hitler and abrogated their 1932 non-aggression pact with Poland, they might never have had to fight Hitler.
But then they couldn’t have stabbed Poland in the back, stolen half of Poland’s territory, gobbled up the Baltics and attacked Finland either, could they?
And the Soviets wouldn’t ally with Britain or France, because they wouldn’t let the Soviets have half of Poland.
So the Soviets decided to make the deal with Hitler, because he would give them half of Poland.