On 28 June 1942, the Wehrmacht began its offensive against Soviet forces opposite of Army Group South, codenamed Case Blue. After breaking through Red Army forces by 13 July, German forces encircled and captured the city of Rostov. Following the fall of Rostov, Hitler split German forces operating in the southern extremity of the southern Russian SFSR in an effort to simultaneously capture the city of Stalingrad and the Caucasus oil fields. The responsibility to take Stalingrad was given to the Sixth Army, which immediately turned towards the Volga River and began its advance with heavy air support from the "Luftwaffe's" "Luftflotte 4". On 7 August, two German panzer corps were able to flank and encircle a Soviet force of 50,000 personnel and approximately 1,000 tanks, and on 22 August German forces began to cross the Don River to complete the advance towards the Volga. The following day, the Battle of Stalingrad began when vanguards of the Sixth Army penetrated the suburbs of the city.