Weather and geography defeat Germany in the east.
No, it all fell on Hitler when he delayed operation Barbarossa 6 weeks to secure his southern flank. He and his garner also were well aware of the Russian winters. Had Barbarossa gone off as planned, it would have been a different story.
The great Russian military leader, General Winter. That and the Soviets trading territory for time to arm up and regroup. And an almost endless manpower supply.