On July 26, Hooker was promoted to major general, to rank from May 5. During the Second Battle of Bull Run, the III Corps was sent to reinforce John Pope's Army of Virginia. Following Second Bull Run, Hooker replaced Irvin McDowell as commander of the Army of Virginia's III Corps, soon redesignated the I Corps of the Army of the Potomac. During the Maryland Campaign, Hooker led the I Corps at South Mountain and at Antietam, his corps launched the first assault of the bloodiest day in American history, driving south into the corps of Lt. Gen. Stonewall Jackson, where they fought each other to a standstill. Hooker, aggressive and inspiring to his men, left the battle early in the morning with a foot wound. He asserted that the battle would have been a decisive Union victory if he had managed to stay on the field, but General McClellan's caution once again failed the Northern troops and Lee's much smaller army eluded destruction. With his patience at an end, President Lincoln replaced McClellan with Maj. Gen. Ambrose Burnside. Although Hooker had criticized McClellan persistently, the latter was apparently unaware of it and in early October, shortly before his termination, had recommended that Hooker receive a promotion to brigadier general in the regular army. The War Department promptly acted on this recommendation, and Hooker received his brigadier's commission to rank from September 20. This promotion ensured that he would remain a general after the war was over and not revert to the rank of captain or lieutenant colonel.