The IX and III Corps, were from McClellan's Army of the Potomac. They were sent as reinforcements for the Army of Virginia and fought with Pope at Second Manassas. The IX Corps commander, Maj. Gen. Burnside, stayed behind with McClellan's Army of the Potomac and his second in command, Maj. Gen. Reno, assumed temporary command of the Corps. When Reno became ill, the First Division commander, Brig. Gen. Issac Stevens, took over tactical control of the Corps.

Adding to the already confused record of the battle, Pope's Army of Virginia ceased to exist as a organizational entity as it entered the Washington defenses. Individual units were reassigned to the Army of the Potomac under the command of Maj. Gen. George B. McClellan and General Pope was sent out west to a distant post.

A Union Corps usually had three division's, each division would have three brigades, and each brigade would have four regiments. Although a regiment could contain 800 - 1000 men, most Civil War combat regiments averaged about 400. The IX Corps was considerably under strength at all levels. However the III Corps was in much better shape, with Kearny's 1st Division, 2nd Brigade, fielding seven regiments under the command of Brig. Gen. Birney.