Germany's war effort took a nose dive the moment Hitler took control away from his generals in '41. They never wanted to invade Russia. They certainly didn't want to capture Stalingrad, a city with little strategic significance.

Hitler didn't know what he wanted in Russia and the Soviets exploited this. He refused to allow his generals to employ the kind of elastic offense and defense that had proven so effective in France and Africa. It was a religious thing for him. He had been warned numerous times about the over-extension of forces in the Caucuses and those officers had been labelled as defeatists.

When Manstein failed to break through the encirclement, he recommended to Paulus that he break out. He could have ordered him, being his superior, but he feared Hitler's wrath. He didn't make that mistake again at Kharkov.

I've studied this subject in some depth and ultimately, I don't see anyway the Germans could have wrestled anything resembling 'success' in Russia. They didn't have the forces to conquer or hold that much territory. Even if Hitler had ignored the Stalingrad bait, even if Typhoon had succeeded in '41, ultimately Soviet production, foreign aid and manpower superiority would have overwhelmed them.

It's really quite amazing that the Germans lasted as long as they did, and without a real war economy!