That must be why the Soviet prosecutor at Nuremberg went nuts when von Weizsacker revealed the existence of the Secret Protocol to the Pact. You know, the one that cynically carved up Eastern Europe.
Stalin cut his deal with Hitler because he believed Britain and France wanted to defeat Germany at the cost of copious amounts of Russian blood. He figured that the European states would exhaust themselves and he would clean up in Europe at little cost. But the French folded and Stalin was left to face Hitler with an army that wasn’t ready because most of the officers had been purged.
And so it happened that Stalin defeated Hitler...at the cost of copious amounts of Russian blood. Yeah, Vlad. It was a heck of a deal.
They were purged because of a clever ruse perpetrated by the Nazis. They used fake radio messages to make Stalin's secret police within the Army think that his officer corps was full of spies and traitors.
Stalin fell for it hook line and sinker, and had roughly one-third of his commanders shot by firing squads.