Organized at Cairo, Ill., and mustered in August 1, 1861, proceeding in the next month to Paducah, Ky., where it remained until February, 1861, during which time it was engaged on occasional expeditions; also in the demonstration against Columbus, and in a reconnoissance toward Fort Donelson. On the 5th of February it embarked for Fort Henry, marching thence to Fort Donelson, where it was engaged in the storming of the works; it lost in this, its first battle, 19 killed, 62 wounded, and 8 missing, out of 612 effective men. It was then in General C. F. Smith's Division, Colonel McArthur commanding the brigade, and Lieutenant-Colonel Chetlain the regiment. At Shiloh, it lost 22 killed, 76 wounded, and 3 missing, out of 329 engaged; at Corinth, 15 killed, 79 wounded, and 15 missing, out of six companies engaged, numbering 12 officers and 262 men. At the latter action it fought in Oglesby's (2d) Brigade, Davies's (2d) Division, Army of West Tennessee. The regiment remained at Corinth during the eight months subsequent to that battle; then, on June 6, 1863, it moved to Pocohontas, Tenn., where it guarded the railroad for a few months. In January, 335 of the men reënlisted, and went home on the customary veteran furlough granted in such cases. The Twelfth started on the Atlanta campaign, May 9, 1864, having been assigned to Mersy's (2d) Brigade, Sweeny's (2d) Division, Sixteenth Corps. After the fall of Atlanta, this division was transferred to the Fifteenth Corps as its Fourth Division. General Corse commanded the division at the defense of Allatoona; General Rice commanded it during the March to the Sea and in the Carolinas.