XXI Corps (Union Army)
Taking part in the Tullahoma Campaign, the XXI Corps was heavily engaged at Chickamauga on September 19–20, 1863, where it was nearly destroyed. It was Rosecrans' mistaken decision to pull Thomas Wood's division of this corps to support George Thomas's XIV Corps that resulted in General James Longstreet's breakthrough against the Union center. It should be added, though, that elements of XXI Corps, most notably William J. Palmer's division, aided Thomas in his successful rearguard action on Snodgrass Hill. Along with the remnants of XX Corps, the corps reorganized into IV Corps shortly after the battle.