Days) and would lose still more men at South Mountain; it is estimated that the corps had 8,000 men at Antietam out of a paper strength of 14,000. The VI Corps was also joined by Darius Couch's division, formerly part of Erasmus Keyes's IV Corps, and now being brought up from the Virginia Peninsula. The V Corps was heavily engaged in the Seven Days Battles and at Second Bull Run, and had lost significant amounts of men; several new regiments of green troops would replace them. A new division of nine month regiments led by Brig. Gen Andrew A. Humphreys was added, but they would not arrive until after Antietam. The IX Corps had had two divisions at Second Bull Run (commanded by General Reno as Burnside was not present at the battle); for the Maryland campaign, it was joined by a third division under Brig. Gen Samuel Sturgis and Brig. Gen Jacob Cox's "Kanawah" Division, on loan from the West Virginia area. It included several green regiments and the corps as a whole was quite inexperienced as Second Bull Run had been the only serious engagement it had fought in. The XII Corps had not fought at Second Bull Run and its last engagement had been at Cedar Mountain a month earlier; some men from this corps were left in the Washington defenses and swapped for a number of green regiments. After Nathaniel Banks was fired on September 12, the senior division commander, Alpheus Williams, commanded the corps for a few days until Maj. Gen Joseph K. Mansfield, an old regular army officer with 40 years of service, was named to command. Confederate General Robert E. Lee's Army of Northern Virginia was organized into two large infantry corps, about 55,000 effectives at the beginning of September. The First Corps, under Maj. Gen. James Longstreet, consisted of the divisions of Maj. Gen. Lafayette McLaws, Maj. Gen. Richard H. Anderson, Brig. Gen. David R. Jones, Brig. Gen. John G. Walker, Brig. Gen. John Bell Hood, and an independent brigade under Brig. Gen. Nathan G. "Shanks" Evans. The Second Corps, under Maj. Gen. Thomas J. "Stonewall" Jackson, consisted of the divisions of Brig. Gen. Alexander R. Lawton, Maj. Gen. A.P. Hill (the Light Division), Brig. Gen. John R. Jones, and Maj. Gen. D.H. Hill. The remaining units were the Cavalry Corps, under Maj. Gen. J.E.B. Stuart, and the reserve artillery, commanded by