Organized at Keokuk, February 22, 1862, and mustered in on March 14th. It left the State, 1,038 strong, on March 19th, stopping at St. Louis where it was armed and equipped, and on the morning of April 6th arrived at Pittsburg Landing just as the battle of Shiloh was commencing. It had been previously assigned to Prentiss's Division, but being unable to find that command, Colonel Reid ordered the regiment into line, and it fought in McClernand's Division. Though entering a battle with so little preparation, it rendered efficient service and acquitted itself creditably. Its loss at Shiloh was 21 killed, 156 wounded, and 8 missing; a total of 185, out of 760 engaged. At the battle of Corinth the Iowa Brigade was commanded by Crocker, and fought in McKean's Division; the Fifteenth, Colonel Belknap commanding, sustained the principal loss in the brigade, its casualties amounting to 11 killed, 67 wounded, and 8 missing, out of "about" 350 engaged.

In the early spring of 1863, the regiment encamped near Lake Providence, La., and assisted in digging the military canal connecting the Lake with the Mississippi River. During the Vicksburg campaign of the following summer, the Iowa Brigade served in Mcarthur's Division of the Seventeenth Corps. The regiment reënlisted, and, returning from its veteran furlough, joined Sherman's Army, June 10, 1864, at Kenesaw, Ga. In the battle of Atlanta--July 21st and 22d--the regiment lost 178 men, killed, wounded, and missing, and captured the flags of two Confederate regiments. During the March to the Sea, and through the Carolinas, the division Fourth Division, Seventeenth Army Corps--was commanded by General Giles A. Smith, and the brigade, by General Belknap.