Still some shameless people may claim that it was the UK and US, which saved the USSR from getting over-run by the Nazis. It was Shitler's biggest mistake in his life. He should have stayed neutral. The Molotov–Ribbentrop Pact was more favorable to the Nazis, than it was to the Germans. But he got arrogant and thought that he could achieve what the Mongols, Ottomans, and Napoleon could never achieve.