WE never signed a Non-Aggression Pact with Adolf, Ivan.
The only reason Stalin survived:
1. Lend-Lease - Nearly all of Stalin’s ammo and more than 70% of his trucks were provided by FDR, whose administration was already riddled with commie spies.
2. SPY - Stalin had a spy in Tokyo named Richard Sorge, who had contacts at the German Embassy there. As allies, Germany and Japan kept one another generally aware of their general intentions. Sorge heard that Japan had no intention to invade the USSR therefore Stalin could transfer half of his troops tied down in a useless guarding action there to Stalingrad and Kursk. For that reason Hitler faced what appeared to be an endless tide of humanity opposing him.
3. Winter - Mud, then froze.
4. Hirler - He meddled in the plans of his generals.
The most critical was maybe #2.