Hampton Roads against the ironclad CSS Virginia in its naval battle against the USS Monitor on March 9, 1862. Until the fall of 1862, Mansfield commanded the Suffolk Division of the VII Corps of the Department of Virginia in the vicinity of Suffolk. During the Maryland Campaign, when Confederate General Robert E. Lee invaded the North for the first time, Mansfield was given command of the XII Corps of the Army of the Potomac, as of September 15, 1862, two days prior to the Battle of Antietam. He arrived in the camp with 40 years of army experience, but no recent combat. He was white-haired and white-bearded, but had a vigorous manner that belied his age. His officers considered him nervous and fussy, but his men, many of whom were new recruits, liked him well enough due to his shows of blustery enthusiasm and fatherly assurance. On the morning of September 17, 1862, the I Corps under Maj. Gen. Joseph Hooker attacked from the north, parallel to the Hagerstown Turnpike, smashing into the Confederate left flank. Mansfield's corps came immediately behind. As the lead brigade moved through an open field east of the Miller farmstead, they were subjected to fire by Confederate gunners, who took a terrible toll on the rookie soldiers. The troops were advancing in column formation, more suitable for marching, and their officers ordered deployment into open battle lines, which would reduce the risk of casualties from artillery shelling. Mansfield countermanded these orders, insisting they stay in column, because he was concerned that outside of the immediate control of their officers, the men would break and run. The result of this was to improve the mass of men that descended on the Confederate lines. Mansfield personally led troops on his left flank, from the brigade of Brig. Gen. Samuel W. Crawford, in the East Woods. He returned to the rear to bring up more troops, and when he reached the line again he saw soldiers from the 10th Maine Infantry regiment firing into the woods. Mansfield, assuming that men from Hooker's corps were in the woods, rode down the regimental line crying out, "You are firing on our own men!" The soldiers convinced Mansfield that in fact they were not and were receiving heavy fire from the woods. Mansfield replied, "Yes, yes, you are right," and just then his horse was hit and a bullet caught him