The battle of stalingradwas a crushing defeat for germany because

The spent insane levels of manpower on the battle, taking from other fronts and stranding the soldiers in the west who had to contend with a coming American invasion of occupied France. The German's then had the added problem of the worlds largest army swarming their eastern front. They were simply doomed because Russian animosity and the now faltering German morale were dominating the eastern front as a result.