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 th