My understanding is that Hitler had to break away an armored division of the invasion force to go to Greece to help out his "ally" the Italians who were getting their @$$es handed to them by the Greeks.
Had that division gone where it was intended, the Germans would have won the battle of Stalingrad and then taken Russia.
So in other words, the Germans lost because their Italian allies wrecked their invasion plan.
Yes the diversion is true, but that did NOT cost them the war in Russia. They never could win that war, and Stalingrad was of no importance other than the name after they destroyed the place.
Hell they had Leningrad covered also, as well as threatening Moscow.
These places where just that, cities in the middle eastern part of a country so vast and in no way could the stupid Nazis win it. And Hitlers generals knew that. The only way he could have done any serious harm is if they took the oil rich areas of southern Russia. Instead, the mental midget, a legend in his own mind Hitler, decided that a few cities were more important.
The idiot never knew how to fight a war. One important rule is that one does NOT win wars by gaining vast amounts of territory. One wins wars by totally and utterly destroying his enemies.
He never destroyed the BEF, all the French, all the Poles, the Russians etc, and then scoffed at the USA when we entered the war. So, who totally and utterly destroyed their enemies?