The 3rd U.S.V.I. was authorized at the Rock Island prisoner camp in October 1864 as a one-year regiment but not organized until February 1865. It was ordered to the Department of the Missouri, arriving at Fort Kearny, Nebraska, April 9, 1865, where it was assigned to duty in the Districts of Nebraska and Colorado. Companies A and B were stationed at Fort Kearney; C and D at Cottonwood, Colorado; E and F at Fort Rankin; and G and H at Julesburg, Colorado, protecting overland mail routes from Indian attacks. Companies I and K were sent to Fort Laramie, and on May 15, 1865, were parceled out in small detachments along 300 miles of the Pacific Telegraph Company line from Laramie to South Pass, Territory of Idaho (now Wyoming). Company I was involved in numerous skirmishes, including combat at the Battle of the Platte Bridge Station, and suffered higher casualties than any other company of the 3rd U.S.V.I. The regiment mustered out November 29, 1865, having served with distinction, including a low desertion rate.