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.
If you remember the movie “Enemy at the Gates” a lot of the Soviet snipers were trained by the Germans in the 30s as well.