XX Corps ( Union Army ) Two corps of the Union Army were called XX Corps during the American Civil War . Though both served in the Union Army of the Cumberland , they were distinct units and should be recognized as such . McCook ' s Corps . The first XX Corps , under the command of Alexander M . McCook , was organized in the aftermath of the Battle of Stones River in January 1863 from what had been the XIV Corps , or right wing of the army , at that battle . It was so identified with its commander that it was generally referred to by other soldiers and even officers as " McCook ' s corps " . The corps took part in a skirmish with Bragg ' s rearguard at Liberty Gap , Tennessee , during the Tullahoma Campaign in June 1863 . It fought its only major battle under this designation at Chickamauga , where it suffered horrendous casualties in the two days of fighting . The corps took heavy casualties , and it ( along with Thomas L . Crittenden ' s XXI Corps ) , became consolidated into the new IV Corps . McCook , blamed in large part for the failure at Chickamauga , was relieved of command . Hooker ' s corps . After the Battle of Gettysburg , with the armies of the east engaged in stalemate ( and a large portion of Robert E . Lee ' s Army of Northern Virginia , under James Longstreet , was serving with Braxton Bragg in Tennessee ) , Washington dispatched Joseph Hooker , discredited after his defeat at the Battle of Chancellorsville the previous May , with the XI and XII Corps of the Army of the Potomac to try to relieve Bragg ' s siege of Chattanooga . The command played a decisive role in the Battle of Wauhatchie , which opened up the " Cracker Line " to the besieged Union army , and seized Lookout Mountain in the famed " Battle Above The Clouds " during the early stages of the Battle of Chattanooga . On April 4 , 1864 , just before the onset of the Atlanta Campaign , William T . Sherman , authorized the consolidation of XI and XII Corps as XX Corps , under Hooker ' s command , to serve in the Army of the Cumberland . Units from XI and XII Corps were combined in each division . The corps fought valiantly throughout the Atlanta Campaign . After James B . McPherson was killed in the Battle of Atlanta on July 22 , Oliver O . Howard was made commander of the army . Hooker , partially because Howard was junior to him and partially because he blamed Howard ( who had commanded XI Corps at the Battle of Chancellorsville , where it had been routed during Stonewall Jackson ' s famed flank march ) for his part in the defeat of the Army of the Potomac , resigned . He was replaced first by Alpheus S . Williams , and then Henry W . Slocum , both former XII Corps commanders . Its troops were the first to enter Atlanta after its surrender on September 1 , and later went with Sherman ' s Army of Georgia during his March to the Sea . Williams commanded it from then until the Carolinas Campaign ( Slocum having been promoted to army command in the mean time ) . It played a major part in the seizure of Savannah in December and was actively engaged throughout the Carolinas Campaign , particularly at Bentonville , where it absorbed the main blow of Joseph E . Johnston ' s counterattack . Because fighting at Bentonville had not destroyed Johnston ' s army , Sherman replaced Williams with the more petulant Joseph A . Mower . It took part in the Grand Review and was disbanded in June 1865 .