Why was a German assault on Stalingrad a crushing defeat for the Germans?

The Germans lacked Hoth's 4th Panzer army which would have helped to capture the city of Stalingrad but instead the 4th Army was assisting Manstein in the Crimean peninsula .

The loss of Paulus' 6th Army at Stalingrad was a crushing defeat for the Wehrmacht which the Germans never recovered from and lost momentum which was never regained and marked a signal victory for the Russians and was a significant morale booster to Russian war efforts .