Definitely, the invasion of USSR. By deconstructing the factories quickly as they backed up, the Russians were able to rebuild the factories behind the frontline. The new weaponry came out, such as La-5 and La-7 - by far, the best fighter-plane and storm-plane of WWII, respectively. I won't even mention T-34 - that's a well-known legend to all. By the time Normandy invasion happened, Russia was already invading Germany and, overall, Russians took off over 85% of the German forces. Thanks to Americans for holding the Japanese off at the Pacific.