And of course it helped that the rubble of the city provided an ample defense against Panzers and provided for one massive bastion, and the Volga a natural barrier against the encirclement tactics of blitzkrieg. Germany lacked the artillery development, the bomber development and the tank development to be able to fight the Soviets under less than ideal conditions.

And I remember seeing in some documentary how Soviet envoys sent to Paulus' HQ after the Soviet counter-offensive were bewildered by the fact that the Germans weren't surrendering.