When the cautious McClellan failed to follow up on Antietam, he was replaced by Maj. Gen. Ambrose Burnside. Burnside was soon defeated at the Battle of Fredericksburg on December 13, 1862, when more than 12,000 Union soldiers were killed or wounded during repeated futile frontal assaults against Marye's Heights. After the battle, Burnside was replaced by Maj. Gen. Joseph Hooker.