The weather certainly played a part in repelling the German invasion. But even worse than the cold winter was the late fall of 1941. At that time russian roads were almost exclusively dirt roads that turned into muddy traps in the rain. These almost impassable roads put the advance towards Moscow almost to a complet stop. And not only did it slow down the advancing troops, but it also made it even harder to supply these troops. And as you already pointed out the German Army was already severely overstretched and arguably undersupplied (German Troops relying on living of the land and no total war measures introduced at the home front yet). Actually when winter came around and the muddy roads were starting to freeze solid the advance towards Moscow was renewed until it was repelled by Siberian reinforcements (with excellent winter equipment). I don't think it was unreasunable to believe that the Axis could defeat the USSR in 1941, considering that Stalin had taken out his military leadership, the Red Army wasn't well equiped and they just fared very poorly against the Finns in the Winter War. If the Wehrmacht had managed to conquer Leningrad, Moscow and Stalingrad, 3 major railways junctions in the western USSR it would have been extremely hard for the Sovjets to reinforce their troops and be able to push back. You have to remember how important the railway system was back then, especially considering the sub-standard condition of the Russian roads. Overall I agree with you that it wasn't just the winter but a greater number of problems that helped defeat the Axis invasion. I really recommend Manstein's memoires of World War II called "Lost Victories" if anyone is interested in the German campaign in the USSR. Of course it's pretty one-sided, but it really illustrates the thoughts of one of the Wehrmacht's best leaders on the eastern front.