Although I am sure we have discussed this somewhere before.

In my opinion it was the encircelment of the German 6th Army and her destrutcion. Although the weather made the positions in stalingrad unbearable, I think had it been summer the 6th army would still have been lost, lack of supplies such as food and ammuntiion cost the troops. I doubt that the Germans attacking moscow would have changed the Germans fate in stalingrad once they were engaged in the city.

Plus as I always say Hitler and Goering are Idiots and were not fit to command anything.

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They shall grow not old as we that are left grow old Age shall not weary them, nor the years condemn
At the going down of the sun and in the morning
We shall remember them. Lest We Forget