of Brig. Gens. Orlando B. Willcox, Samuel D. Sturgis, and Isaac P. Rodman, and the Kanawha Division, under Brig. Gen. Jacob D. Cox. The XII Corps, under Maj. Gen. Joseph K. Mansfield, consisted of the divisions of Brig. Gens. Alpheus S. Williams and George S. Greene, and the cavalry division of Brig. Gen. Alfred Pleasonton. During the march north into Maryland, McClellan changed his army's command structure, appointing commanders for three "wings": the left, commanded by William B. Franklin, consisted of his own VI Corps plus the division of Darius Couch; the center, under Edwin Sumner, consisted of his II Corps and the XII Corps; the right, under Ambrose Burnside, consisted of his IX Corps (temporarily commanded by Maj. Gen. Jesse L. Reno until he was killed at South Mountain) and the I Corps. This wing organization was revoked just before the start of the Battle of Antietam. The army that McClellan took into Maryland was not an entirely cohesive or battle-ready fighting force. At its core were the Peninsula veterans of the II, V, and VI Corps, but a large portion of the army were untested rookie regiments or troops who had never fought as part of the Army of the Potomac. Some of the rookies had never even loaded their muskets, and others were unknowingly armed with defective weapons. The "core" Army of the Potomac, the II, III, V, and VI Corps, which had served through the Peninsula Campaign, were the easiest ones to get ready for action. In addition there were the three corps that had comprised Pope's army. Aside from the Pennsylvania Reserves, which had born the brunt of action in the Seven Days Battles, none of these troops had served under McClellan before, all had been poorly led, and did not have much record of battlefield success. The two divisions of Burnside's IX Corps were also new to McClellan, but they had served capably in North Carolina early in the year. The IX Corps was also being joined by Brig. Gen Jacob D. Cox's "Kanawah" division from West Virginia, new to McClellan as well. These troops had seen no major action so far and were essentially green. The I Corps had excellent troops; as mentioned above the Pennsylvania Reserves had been in the thick of the Seven Days Battles and all three divisions were heavily engaged at Second Bull Run. McDowell however had been written