On November 7, 1862, President Lincoln relieved McClellan of command because of his failure to pursue and defeat Lee's retreating army from Sharpsburg. Ambrose Burnside, despite his indifferent performance as a corps commander at Antietam, was appointed to command the Army of the Potomac. Once again, Lincoln pressured his general to launch an offensive as quickly as possible. Burnside rose to the task and planned to drive directly south toward Richmond. He hoped to outflank Robert E. Lee by quickly crossing the Rappahannock River at Fredericksburg and placing himself in between the Confederate army and their capital. Administrative difficulties prevented the pontoon bridging boats from arriving on time, and his army was forced to wait across the river from Fredericksburg while Lee took that opportunity to fortify a defensive line on the heights behind the city. Rather than giving up or finding another way to advance, Burnside crossed the river and on December 13, launched massive frontal assaults against Marye's Heights on Lee's left flank. His attacks were more successful on Lee's right, briefly breaking through Jackson's line; but due to a misunderstanding continued to pound the fortified heights with waves of attacks, believing that this would enable the troops opposite Jackson to exploit their advantage. The Union Army lost over 12,000 men that day; Confederate casualties were approximately 4,500.O'Reilly, pp. 2–3, 44–48, 498–99.
