McClellan failed to respond in any measurable way to Lee's attempt to invade the North at Antietam led to his replacement by Maj. Gen. Ambrose Burnside. Burnside led Union Army troops in the Battle of Fredericksburg, where they were defeated on December 13, 1862. Over 12,000 Union soldiers were killed or wounded during futile attempts by Union troops to launch frontal assaults against Marye's Heights. After the battle, Burnside was replaced by Maj. Gen. Joseph Hooker.