The origin of the Army of the Cumberland dates back to the creation of the Army of the Ohio in November 1861, under the command of Brig. Gen. Robert Anderson. The army fought under the name Army of the Ohio until Maj. Gen. William S. Rosecrans assumed command of the army and the Department of the Cumberland and changed the name of the combined entity to the Army of the Cumberland. When Rosecrans assumed command, the army and the XIV Corps were the same unit, divided into three "grand divisions" (wings) commanded by Alexander McCook (Right Wing), George H. Thomas (Center), and Thomas L. Crittenden (Left).
General Orders No. 168 was the order passed by the Union Army on October 24, 1862, that called for the commissioning the XIV Corps into the Army of the Cumberland.
The army's first significant combat under the Cumberland name was at the Battle of Stones River. After the battle the army and XIV Corps were separated. The former Center wing became XIV Corps, the Right wing became XX Corps, and the Left wing became XXI Corps. Rosecrans still retained command of the army. He next led it through the Tullahoma Campaign and at the Battle of Chickamauga, after which the army became besieged at Chattanooga. Maj. Gen. Ulysses S. Grant arrived at Chattanooga. Reinforcements from the Army of the Potomac and the Army of the Tennessee also arrived. Rosecrans had been a popular and respected commander, but because of his defeat at Chickamauga and inability to lift the Confederate siege, Grant chose to replace him with George H. Thomas on October 19, 1863.
In the Battles for Chattanooga, Grant had been leery of using the Army of the Cumberland in the main fighting, fearing their morale to be too low after the defeat at Chickamauga. Instead, he used the veterans from the Army of the Potomac, proud of their recent victory at the Battle of Gettysburg, to take Lookout Mountain and planned to use the troops from the Army of the Tennessee, also recent victors at the Siege of Vicksburg, to attack the Confederate right flank on Missionary Ridge. The Army of the Cumberland was given the minor task of seizing the rifle pits at the base of Missionary Ridge. However, once they achieved their objective, four divisions (one led by Philip H. Sheridan) stormed up the ridge and routed the Confederate center. When Grant angrily asked who had ordered those troops up the ridge both Thomas and Gordon Granger, a corps commander in the army, responded they did not know. Granger then replied, "Once those boys get started, all hell can't stop 'em."