 In March 1862, Calhoun enlisted in the Confederate States Army and was commissioned as captain of Company K of the 42nd Georgia Infantry.He served in Knoxville, Tennessee, and then in the Vicksburg Campaign, where he and his regiment were surrendered by John C. Pemberton on July 4, 1863.After being exchanged, Calhoun served in the Atlanta Campaign and was wounded at the Battle of Resaca.