On the 7th of November, 1862, General Rosecrans divided the Army of the Cumberland -- then known as the Fourteenth Army Corps -- into the Right Wing, Center, and Left Wing. The organization of the left wing, as then arranged, remained unchanged until January 9, 1863, when, by authority of the War Department, General Order No. 9, its designation was changed to that of the Twenty-first Corps. No other change was made, the different brigades and divisions remaining as before.
The left wing, or Twenty-first Corps, was organized with Major-General Thomas L. Crittenden in command, and contained the three divisions of W. S. Smith, Van Cleve, and Hascall. At the battle of Stone's River the divisions were commanded by Generals Thos. J. Wood, Palmer, and Van Cleve. The three divisions contained three brigades each; in all, 38 regiments of infantry, and 8 batteries of light artillery. The losses of the left wing at Stone's River amounted to 650 killed, 3,006 wounded, and 873 missing; total, 4,529, out of 12,909 officers and men engaged.
After this battle the enemy fell back, whereupon Rosecrans' Army occupied Murfreesboro, remaining encamped there, or in its vicinity, until June, 1863, when another forward movement took place which ended in the battle of Chickamauga and in the permanent occupation of Chattanooga. The Twenty-first Corps fought at Chickamauga under the same corps and division generals as at Stone's River. The organization was the same, 3 divisions of 3 brigades each; the regiments, however (38 in number), had diminished in size. The corps numbered, at Chickamauga, 14,040 present for duty. Wagner's (2d) Brigade, of Wood's (1st) Division, was not engaged, having been left on duty at Chattanooga. Two regiments were also detailed elsewhere, leaving 11,480 men in action. Of this number, the corps lost 322 killed, 2,382 wounded, and 699 missing; total, 3,403.
There seems to be a general impression that, after the disaster at Chickamauga, the day was saved solely by Thomas' Corps. In justice to the gallant men of Crittenden's command, it should be stated that Palmer's Division of the Twenty-first Corps fought with Thomas during the whole battle; and that General Wood with two brigades of his own division, and one from Van Cleve's which was not cut off, went to the aid of Thomas on the second day.
Soon after this battle the Twentieth and the Twenty-first Corps were consolidated, forming the Fourth Corps. General Crittenden was left without a command, but was subsequently assigned to a division in the Ninth Corps, Army of the Potomac, while on the Wilderness campaign of the following spring.