Organized at Leavenworth in May, 1861, and in June, was ordered into Missouri where it joined General Lyon's forces. It fought at Wilson's Creek, August 10, 1861, a desperate battle in which General Lyon was killed, and in which the regiment suffered an unusual loss, its casualties amounting to 77 killed, 187 wounded, and 20 missing; a total of 284, out of 644 engaged; four line officers were killed, and Colonel Deitzler was severely wounded. General Sturgis, after having passed through many of the bloodiest battles of the war, once remarked that "for downright, hard, persistent fighting, Wilson's Creek beat them all." After this battle the regiment was stationed in Missouri until May, 1862, when it was ordered to Corinth. The summer of 1862 was spent in opening and guarding the Mobile & Ohio R.R. In January, 1863, it moved to Young's Point, opposite Vicksburg, where the regiment was mounted by order of General Grant. It served as mounted infantry during the ensuing eighteen months, including the siege of Vicksburg, after which it joined the expedition to Natchez. During this time it was engaged, almost continuously, on scouting and outpost duty. In October, 1863, it returned to Vicksburg, and was stationed at Black River Bridge, twelve miles south of the city, where it went into winter quarters. The spring of 1864 was also passed in the vicinity of Vicksburg. On June 1, 1864, the regiment, excepting two companies of reënlisted men and recruits, embarked on transports for home, its term of service having expired. While passing Columbia, Ark., the steamer was fired on by an eight-gun battery, killing two of the men. The two companies remaining in the field were continued in service until August 30, 1865, when they were mustered out.