In September 1942, the German Sixth Army under General Friedrich Paulus was on the outskirts of Stalingrad, expecting to take the city within a few days. But Soviet leader Joseph Stalin, realizing the loss of Stalingrad would allow the Germans to advance into the Caucasus, committed significant reinforcements and supplies for defense of the city.
The ensuing desperate battle for Stalingrad involved fierce urban street fighting with tremendous losses on both sides. In November 1942, Marshal Georgy Zhukov launched a counteroffensive and encircled the stalled Wehrmacht 6th Army. In December 1942, a major relief operation failed and promised Luftwaffe air support did not materialize.
In January 1943, General Paulus requested permission for a break-out withdrawal (probably still possible). In response, Hitler promoted Paulus to Generalfeldmarschall and ordered him hold ground at all costs.
On February 2, 1943, General Paulus, with ~150,000 dead soldiers, surrendered the remaining 91,000 men of his army. At the end of the war, only 6,000 soldiers of the 6th army returned home.
With ~2 million combined civilian and military deaths, the battle of Stalingrad was one of the bloodiest battles in history. Like the Battle of Midway for the Japanese, the disastrous Battle of Stalingrad proved to be a major turning point for Germany in WWII.