Thomas was promoted in rapid succession to be lieutenant colonel (on April 25, 1861, replacing Robert E. Lee) and colonel (May 3, replacing Albert Sidney Johnston) in the regular army, and brigadier general of volunteers (August 17). In the First Bull Run Campaign, he commanded a brigade under Maj. Gen. Robert Patterson in the Shenandoah Valley, but all of his subsequent assignments were in the Western Theater. Reporting to Maj. Gen. Robert Anderson in Kentucky, Thomas was assigned to training recruits and to command an independent force in the eastern half of the state. On January 18, 1862, he defeated Confederate Brig. Gens. George B. Crittenden and Felix Zollicoffer at Mill Springs, gaining the first important Union victory in the war, breaking Confederate strength in eastern Kentucky, and lifting Union morale.