The Battle of Stalingrad was both the largest battle in human history in terms of the number of men who fought, in addition to the bloodiest battle in terms of sheer casualties as well. In 1942, the Germans had lost the Battle of Moscow the previous winter. Rather than continue their offensive, they decided to strike south towards Stalingrad in order to gain control of the Caucasus oil reserves.
The German Luftwaffe bombed Stalingrad into submission while the 6th Army surged forward, flanked by the Italian, Romanian, and Hungarian armies. Some of the fiercest combat of World War II took place in the rubble of Stalingrad. The Soviet resistance was fierce, but the 6th Army pushed them back to the Volga River.
In December, just as the Germans were on the verge of victory, outside Soviet armies launched counteroffensives on the flanking Italian, Hungarian, and Romanian armies. These armies broke apart and the Soviets surrounded the 6th Army in Stalingrad before closing in from all sides. Hitler ordered the 6th Army to hold out and be resupplied by the Luftwaffe while outside forces would attempt to break them out.
The Soviets were able to successfully shoot down enough aircraft that the 6th Army could not be resupplied while also stopping the outside German offensive. A month later, the battered 6th Army, out of food, water, ammunition, and trapped in the freezing cold in summer clothing, was forced to surrender.
The Soviets sustained nearly 1,200,000 casualties at Stalingrad, of whom nearly 500,000 were killed. The Axis lost over 800,000 men, including the entire German 6th Army, which was a blow they would never fully recover from.