On January 9, 1863, the U.S. War Department issued General Orders, No. 9, dividing the Army of the Cumberland into three army corps.
General Orders, No. 9
War Dep’t, Adjutant General’s Office
Washington, January 9, 1863
By direction of the President, the Army of the Cumberland, under the command of Major General Rosecrans, is divided into three Army Corps, to be known as the Fourteenth, Twentieth, and Twenty-first.
Major General G.H. Thomas is assigned to the command of the Fourteenth Corps;
Major General A. McD. McCook, to command the Twentieth; and
Major General T.L. Crittenden, to the command of the Twenty-first Corps.
By Order of the Secretary of War:
E.D. TOWNSEND, Assistant Adjutant General.