The only single thing that saved Russia in history is the vastness of the country. And that is why Hitler screwed it up. He was to dam stupid to understand why.
Anyone who intends to invade and take over that hole will have to do it from the middle and bottom up and not west to east or east to west.
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.