The Soviet pact with Hitler was hardly benign. German aircrews trained in the Soviet Union to avoid the rearmament restrictions. Stalin provided critical materials to Germany that also aided in the rearmament. And while the Nazis were rounding up the Jews, the Russians marched the Polish elite into Katy forest to be massacred.
An ironic aspect of Stalin's actions was to pull the Soviet defenses out of their established bog land positions which mad them even more venerable during Barbarossa.
The USSR aided the Weimar Republic not Nazi Germany.
Right away we get into Katyn and the typical knee-jerk equalization of Hitler's death camps with Stalin's crimes, which is another way of stating the extreme right's wish for an alliance with Nazi Germany instead of the Stalin's USSR.
If the West hadn't caved in to Hitler at Munich, and if Poland would have formed an alliance with the USSR instead of helping itself to parts of Czechoslovakia...if, if , if...but the end result is that Stalin gave up on the Alliance which was to have prevented Munich and the destruction of Czechoslovakia by the Nazis, and became neutral and helped himself to parts of Poland...what goes around comes around.