Debate is fervent on this subject: Why was Operation Barbarossa, The Battle of Moscow, and ultimately the War on the Eastern Front a failure for Hitler and a victory for the Soviet Union?

Had Hitler bitten off more than he could chew in fighting from the Baltic Sea in the north to the Black Sea in the South? Were campaigns fought throughout the winter too much for German supply lines stretched too thin?

Or were the Soviet Union's sheer numbers too overwhelming for Germany? Had Lend Lease from the United States played a key role? Or were all factors critical and decisive?

What do you think?