He helped form the Lancaster unit that became Co. K of the Seventh Regiment, Wisconsin Volunteer Infantry. When the unit was Federalized, he entered the Union Army as a lieutenant, and was promoted to captain, August 30, 1861. The Seventh Wisconsin was part of the famed "Iron Brigade of the West." Due to the high casualty rate among its officers, Callis led the regiment at the Battle of South Mountain, Antietam, and several other engagements. He was promoted to major on January 5, 1863. He was shot in the chest on the first day at Gettysburg and lay on the battlefield until the Confederate withdrawal three days later. After a lengthy recovery, he rejoined the Army and was appointed by President Abraham Lincoln military superintendent of the War Department at Washington, D.C., in 1864. He was promoted to lieutenant colonel February 11, 1865, and was subsequently granted a double brevet to brigadier general of volunteers.
