As the start of the winter push to remove the Germans from Stalingrad, the Red Army conducted Operation Uranus to encircle the German 6th Army, which had become embedded in the city after months of fighting. On 19 November the operation began with a heavy attack focussed on the Romanian troops that had been positioned to guard the German flanks concentrated on fighting near the city itself. The less well-equipped Romanian troops were not able to stave off the attack and by 22 November the Russians had completely enveloped over 250,000 men of the 6th Army who occupied Stalingrad and the surrounding area. In an attempt to keep trapped armies alive, supplies had to be dropped over the city by the Luftwaffe. Operation Winter Storm was devised to break through the Soviet lines to the southwest of the city using the 4th Panzer Army (12–19 December), however it proved unsuccessful and by 24 December the Germans were in retreat.