XII Corps (ACW)

XII Corps
XII Corps, 1st Division Badge
Active	1862–1864
Country	United States
Branch	United States Army
Part of	Army of the Potomac
Engagements	First Battle of Kernstown First Battle of Winchester Battle of Cedar Mountain Battle of Antietam Battle of Chancellorsville Battle of Gettysburg Battle of Wauhatchie Battle of Lookout Mountain
Commanders
Notable commanders	Joseph K. Mansfield Alpheus S. Williams Henry W. Slocum

The XII Corps (Twelfth Army Corps) was a corps of the Union Army during the American Civil War.

The corps was formed by U.S. War Department General Order of March 13, 1862, under which the corps organization of the Army of the Potomac was first created. By that order, five different corps were constituted: one of which, composed of the divisions of Alpheus S. Williams and James Shields and commanded by Maj. Gen. Nathaniel P. Banks, was designated as the V Corps. These divisions were then operating in the Shenandoah Valley. On June 26, President Abraham Lincoln ordered that "the troops of the Shenandoah Department, now under General Banks, shall constitute the Second Army Corps" of the Army of Virginia. On September 12, General Order 129, it was ordered that its designation be changed to that of the XII Corps, and that Maj. Gen. Joseph K. Mansfield be placed in command.

The XII Corps was small—only two divisions instead of the customary three—but was composed of excellent material. Among its regiments were the 2nd Massachusetts, 7th Ohio, 5th Connecticut, 13th New Jersey, 107th New York, 28th Pennsylvania, 46th Pennsylvania, 3rd Wisconsin, and others equally famous as crack regiments; all of them with household names in the communities from which they were recruited.

The corps had done considerable hard fighting under its former title. Shields's Division won a victory over Thomas J. "Stonewall" Jackson at Kernstown on March 23, and Williams' Division fought well at Winchester, May 25, while on Banks' retreat. The Battle of Cedar Mountain was also fought by this corps, alone and unassisted; and, although defeated by the overwhelming force of the enemy, the record shows that the two divisions did some of the best fighting of the war there. In that battle the divisions were commanded by Generals Williams and Christopher C. Augur; loss, 302 killed, 1,320 wounded, and 594 missing; total, 2,216, out of less than 6,000 engaged. This loss fell on four brigades, Samuel W. Crawford's Brigade losing 867 men out of 1679, reported by Crawford as "present in engagement". At the Second Battle of Bull Run the corps was held in reserve.