This corps was identical with that part of the Army of the Cumberland, or Fourteenth Corps, which had been designated as the Right Wing until January 9, 1863, when, under General Order No. 9, the War Department directed that the Right Wing be set apart and designated as the Twentieth Corps. Major-General McCook, its former commander, and Generals Davis, Johnson, and Sheridan, its former generals of division, were retained in command.
The troops composing "McCook's Corps," as it was generally called in the army, were veterans who had withstood the fire of hard-fought fields. Many of the regiments had fought at Shiloh, and at Chaplin Hills, and all of them were engaged at Stone's River. McCook had distinguished himself at Shiloh, where his division contributed largely to Buell's success in the second day's battle; also at Chaplin Hills, in which battle his command was almost the only force engaged. The Twentieth Corps, at the time it was so designated, contained the same troops which fought at Stone's River, with the same organization of divisions and brigades. There were three divisions, each containing three brigades; in all, 37 regiments of infantry, and 9 batteries of light artillery, one battery being attached to each brigade. It numbered, just before the battle of Stone's River, 13,779 present for duty.
After Rosecrans' victory at Stone's River, the Twentieth Corps advanced with the Army of the Cumberland and occupied Murfreesboro, where it remained until June, 1863. The advance on Chattanooga then commenced, during which the corps encountered the enemy at Liberty Gap, Tenn., on the 25th of June. Its casualties in that action amounted to 42 killed, 231 wounded, and 1 missing; total, 274.
It accompanied Rosecrans across the Cumberland Mountains in his pursuit of Bragg, and on Sept. 19th fought at Chickamauga. In this battle McCook's Corps took eight brigades, 12,480 men, into action; it lost 423 killed, 2,698 wounded, and 1,215 missing; total, 4,336. One brigade -- Post's (1st) Brigade, Davis' (1st) Division--was not engaged, being absent guarding a supply train.
On September 28th, 1863, the Twentieth and Twenty-first Corps were consolidated, forming the Fourth Corps, Army of the Cumberland. By this arrangement General McCook was left without a command.