Russians also started the war when they signed the Non-Aggression pact with Hitler, never forget that.
The Soviets invaded Poland with the Germans in 1939, but France & Britain apparently didn’t feel strongly committed enough to their treaty to declare war on Russia along with Germany. I’m sure when the Soviets killed all of those Poles in Katyn they knew there would be no repercussions (and there haven’t been).
Because of military restrictions imposed by Versailles, the Soviets trained many German pilots in the 1930s until Hitler’s “coming out” party; they were birds of a feather (national socialism & international socialism). All that was in question was who got to rule what; that was the cause of their antagonism.