During August and September, as the battle for the city of Stalingrad progressed, 21st Army maintained the pressure on Sixth Army's positions on the right bank of the Don. During October the Rumanian Third Army replaced German divisions defending the northern sector of the Don bend, but the Rumanians were unable to prevent 21st Army from further enlarging the Serafimovich bridgehead. Soviet high command had decided to use the Serafimovich bridgehead to launch a major offensive (codenamed Uranus) to the southeast which, in conjunction with an offensive from Soviet positions south of Stalingrad, would strike into the flanks of Sixth Army to end the siege of the city. At the beginning of November Major-General Ivan Chistyakov replaced Danilov as commander of 21st Army. (Penskovskii, promoted in October to the rank of Major-General, remained as the army's chief of staff). Subsequently, 21st Army was reinforced with a tank corps and an elite cavalry corps. By then the army's bridgehead on the right bank of the Don was 50 kilometres long stretching from west of Serafimovich to east of Kletskaya. During November, 21st Army relinquished control of the Serafimovich sector of the bridgehead to the newly arrived 5th Tank Army. This tank army and the mobile forces of 21st Army were to spearhead the encirclement drive from the north.