The classic double envelopment on a massive scale. While the Germans were fixated on the city of Stalingrad, Zhukov secretly massed over a million men on their weak flanks.
The Germans believed the Red Army was exhausted. Zhukov fed their delusion by feeding only small units into the meat grinder of the city, keeping his main force hidden in the steppe.
On November 19, 1942, in freezing fog, 3,500 Soviet guns opened fire. The Romanian armies on the German flanks collapsed instantly. Four days later, the two Soviet pincers met at Kalach, trapping 300,000 German soldiers.
Context
This was the first time in history a German field army had been destroyed. The myth of invincibility was broken.