From the Peninsula to Maryland: Burnside's role in the summer of 1862
As a result of his success in North Carolina, Ambrose Burnside was promoted to Major General of volunteers on March 18, 1862. Following George McClellan's failure to take Richmond during the Peninsula Campaign, Burnside was offered command of the Army of the Potomac. He refused, citing his loyalty to McClellan and his own lack of military experience.
As a result of this decision McClellan retained command and Burnside detached part of his corps to support John Pope's Army of Virginia in the Northern Virginia Campaign. After the Union debacle at the Second Battle of Manassas (Bull Run), Burnside was again offered command of the Army of the Potomac and again refused, which helped influence Lincoln to decide to retain McClellan after the Army of Virginia was consolidated into the Army of the Potomac.
When the Maryland Campaign began, Burnside was given command of the "Right Wing" of the Army of the Potomac, consisting of the I and IX Corps. He performed superbly in his initial mission of keeping the Confederate from advancing on Baltimore. He was then sent to action at the Battle of South Mountain. Following this battle McClellan separated the I and IX corps as the Army of the Potomac took position on the field of Antietam, placing them on opposite ends of the Union battle line. This effectively eliminated Burnside's role as right wing commander.
Burnside was hesitant to relinquish his position of higher authority, and funneled his orders to the IX Corps (his normal command) through General Jacob D. Cox (who was acting as corps commander while Burnside was commanding the right wing) rather than issuing them directly to the IX Corps divisions himself. This cumbersome arrangement helped to slow his efforts to execute his orders to turn the right flank of the Confederate line during the Battle of Antietam.
McClellan's battle plan called for the Army of the Potomac to strike the Confederates on both sides of their defensive line simultaneously, and when the fighting began it seemed as though this plan might succeed. Even as the Union assaults were being made on the sunken road, a mile and a half farther south Burnside began his attack on the Confederate right.
Burnside recognized the importance of finding a crossing other than the bridge that would come to bear his name. He sent Rodman's division downstream with the purpose of doing precisely that. Unfortunately the ground had not been properly surveyed before the advance of Rodman's division and it took nearly three hours to find a suitable crossing.
During this interval McClellan sent repeated orders to Burnside to push through and take the bridge. One such order included the missive, "Tell him if it costs 10,000 men he must go now." When McClellan sent his inspector general to confront Burnside, the latter reacted indignantly: "McClellan appears to think I am not trying my best to carry this bridge; you are the third or fourth one who has been to me this morning with similar orders." In accordance with McClellan's orders, while Rodman was attempting to find a crossing Burnside focused his attention on the bridge. Despite multiple assaults by Burnside, however, Robert Toombs' Georgia Brigade on the high ground above the bridge succeeded in delaying the Union advance for three hours.
At about 1:00 pm, Burnside's men finally succeed with a consolidated push by the 51st Pennsylvania and 51st New York infantry regiments, driving the Confederates back and securing the bridge.
Once Burnside's men had gained the crossing at the bridge it took two additional hours to get all the men across and organized before a general advance could continue. This additional delay allowed just enough time for A.P. Hill's Confederate division to come up from Harpers Ferry and repulse the Union breakthrough.
Once engaged with Hill Burnside requested reinforcements from McClellan, and when McClellan refused the battle ended in a tactical stalemate.
Despite criticism for his slow response and the delay in moving his men forward, Lincoln still believed that Burnside was his best choice for command of the Army of the Potomac, and when he relieved McClellan in November, Burnside was ordered to accept the command he had twice declined.