From Wikipedia

The Great Battle On The Volga

Operations Uranus, Saturn and Mars: Soviet advances on the Eastern Front, 18 November 1942 to March 1943:      to 12 December 1942      to 18 February 1943      to March 1943 (Soviet gains only)

Main articles: Battle of Stalingrad, Operation Saturn, Second Rzhev-Sychevka offensive, Third Battle of Kharkov, and Battle of Velikiye Luki

While the German 6th Army and 4th Panzer Army had been fighting their way into Stalingrad, Soviet armies had congregated on either side of the city, specifically into the Don bridgeheads that the Romanians had been unable to reduce, and it was from these that they struck on 19 November 1942. In Operation Uranus, two Soviet fronts punched through the Romanians and converged at Kalach on 23 November, trapping 300,000 Axis troops behind them. A simultaneous offensive on the Rzhev sector known as Operation Mars was supposed to advance to Smolensk, but was a failure, with German tactical flair winning the day.

The Germans rushed to transfer troops to Russia for a desperate attempt to relieve Stalingrad, but the offensive could not get going until 12 December, by which time the 6th Army in Stalingrad was starving and too weak to break out towards it. Operation Winter Storm, with three transferred panzer divisions, got going briskly from Kotelnikovo towards the Aksai river but became bogged down 65 km (40 miles) short of its goal. To divert the rescue attempt the Soviets decided to smash the Italians and come down behind the relief attempt if they could, that operation starting on 16 December. What it did accomplish was to destroy many of the aircraft that had been transporting relief supplies to Stalingrad. The fairly limited scope of the Soviet offensive, although still eventually targeted on Rostov, also allowed Hitler time to see sense and pull Army Group A out of the Caucasus and back over the Don.

On 31 January 1943, the 90,000 survivors of the 300,000-man 6th Army surrendered. By that time the Hungarian 2nd Army had also been wiped out. The Soviets advanced from the Don 500 km (300 miles) to the west of Stalingrad, marching through Kursk (retaken on 8 February 1943) and Kharkov (retaken 16 February 1943). In order to save the position in the south, the Germans decided to abandon the Rzhev salient in February, freeing enough troops to make a successful riposte in eastern Ukraine. Manstein's counteroffensive, strengthened by a specially trained SS panzer corps equipped with Tiger tanks, opened on 20 February 1943, and fought its way from Poltava back into Kharkov in the third week of March, upon which the spring thaw intervened. This had left a glaring bulge in the front centered on Kur