Don Carlos Buell
With the outbreak of the Civil War, Harker was assigned to duty in Ohio to help train newly recruited volunteer soldiers. He was promoted to first lieutenant of the 15th U.S. Infantry on May 14, 1861, and then to captain on October 24, 1861. He was subsequently appointed as colonel of the 65th Ohio Infantry, a regiment in the forces of Brig. Gen. Don Carlos Buell. He became the regiment's colonel on November 11, 1861.