Basically the Russians fought bravely, outnumbered the Germans by quite a bit and had short supply lines. On the German side Hitler prevented his Field Marshall Paulus from maneuvers which could have saved the army and made them just keep on until they just couldn’t go any more.
I remember Churchill saying that Stalin was demanding the British attack somewhere, maybe in the Middle East with 30 divisions. Churchill said they had just scraped together a single division to send to Malta.
He later learned that Stalin had 100 divisions training and equipping East of the Urals. With such a massive army they knew they would win.
Sorry for the double post.
Hitler made a critical mistake after surviving the winter outside Moscow. Split Army Group South to go after both Stalingrad and the oilfields to the south instead of concentrating on the fuel. Also did not help that the Germans were so brutal to the civilian population as they moved east. Germans bit off more than they could chew. Overextended, under supplied, arrogant in their assumptions.