In an attempt to resolve their persistent shortage of oil, the Wehrmacht had launched "Fall Blau" (Case Blue), a massive offensive aimed against the Caucasian oilfields, in 1942. After German air attacks, the 6th Army, led by Friedrich Paulus, was tasked with capturing Stalingrad, a key city on the River Volga. His troops, supported by 4th Panzer Army, entered the city on 12 September. Hand-to-hand combat and street fighting ensued. The Red Army launched a huge counteroffensive on 19 November, codenamed Operation Uranus, which was designed to encircle the German armies and trap them in the city; this goal was accomplished on 23 November. Hitler, aware that if Stalingrad were lost it would likely never be retaken, appointed Manstein as commander of the newly created Army Group Don ("Heeresgruppe Don"), tasked with mounting a relief operation named "Unternehmen Wintergewitter" (Operation Winter Storm), to reinforce the German hold on the city. Manstein's initial assessment on 24 November was that the 6th Army, given adequate air support, would be able to hold on.