Organized at Burlington, in July, 1861, leaving the State on August 11th. During the rest of the year and in the following winter it was on active duty in Missouri. In March, 1862, it engaged in the operations around New Madrid, Mo., after which it was stationed for a few months in various places in the Southwest. In August, 1862, it encamped at Jacinto, Miss., leaving there, September 18th, for Iuka, where it fought the next day under Rosecrans. It was then in Sanborn's (1st) Brigade of Hamilton's Division, and sustained the heaviest loss of any regiment in that battle, its casualties amounting to 37 killed, 179 wounded, and 1 missing; among the killed were five line officers. General Rosecrans said officially, that "the glorious Fifth Iowa bore the thrice-repeated charges of the rebel left with a valor and determination seldom equalled, never excelled by veteran soldiers." During the Vicksburg campaign the Fifth was in Boomer's (3d) Brigade, Crocker's Division, Seventeenth Corps, its hardest fighting occurring at Champion's Hill, where it lost 19 killed and 75 wounded out of 350 engaged. It marched to Tennessee in November, 1863, where it fought at Missionary Ridge; its casualties in that battle were 2 killed, 22 wounded, and 82 captured or missing. The reenlisted men received the usual veteran furlough of one month, and went home in April, 1864. Returning, they arrived at Decatur, Ala., on May 14th, after which the regiment was stationed at Huntsville, Ala., at Kingston, Ga., and at other places, until August, 1864, when it was mustered out. The recruits and reenlisted men who were retained in the field, were transferred to the Fifth Iowa Cavalry. Colonel Worthington was killed on the picket line at Corinth, May 22, 1862, having been shot by mistake.