Mustered in at Springfield August 10, 1861. On the 13th it moved to Jefferson Barracks, Mo., and thence to Paducah, Ky., where it encamped during the winter. In March, 1862, the regiment embarked for Pittsburg Landing, where, three weeks later, it took part in the battle of Shiloh. The Fortieth was then in Sherman's Division; its loss at Shiloh was 47 killed, 160 wounded, and 9 missing; total, 216. The regiment received the compliments of General Sherman for its gallantry in this battle, particularly for the steadiness with which it remained in line when requested by him to do so, although its cartridge boxes had been emptied and the enemy were in its immediate front. The remainder of the year 1862, and all of 1863, was passed in the vicinity of Corinth, Vicksburg and Memphis; during the Vicksburg campaign it served in Hicks's (2d) Brigade, W. S. Smith's (1st) Division, Sixteenth Corps. This division was transferred in September, 1863, to the Fifteenth Corps, becoming the Fourth Division, General Hugh Ewing, under whose command it fought at Missionary Ridge. Only five companies of the Fortieth were engaged in that battle,— Companies A, C, E, I and G; the other five had been temporarily detached, and were serving as mounted infantry; the five companies engaged, numbering 130 men, lost 6 killed, 42 wounded, and 1 missing. The regiment accompanied the Fifteenth Corps on the Atlanta campaign, leading one of the assaulting columns at Kenesaw, where Lieutenant-Colonel Rigdon S. Barnhill was killed. After the fall of Atlanta the Fortieth was transferred to the First Division, General Charles R. Woods commanding, with which it marched to the Sea and through the Carolinas.