Spurned allies with the Nazis, the Russian retaliated by having more people to catch them than the Nazis had bullets. The overwhelming numbers of dead Russians allowed the mainly US and British forces to advance against the Nazi war machine from the west. Neither the Russians nor the other allies would have defeated the Nazis in anywhere the same timeframe, if at all, without the other half.
Exactly right, and had the war in Europe lasted just a few months longer, the Hiroshima Bomb would have landed on Berlin instead.