At the beginning of the American Civil War in 1861, Campbell chose to follow his home state and the
Union cause. He volunteered for service in the state forces on May 29, and was assigned as a captain of Battery A in the Pennsylvania Light Artillery. On August 5 Campbell was promoted to
lieutenant colonel in the Pennsylvania Artillery, and to
colonel on September 13. Joining the Union Army in 1862, Campbell was appointed to command the
57th Pennsylvania Infantry as its colonel. He led the 57th Infantry, part of
Brig. Gen. Philip Kearny's division, in the
Peninsula Campaign of 1862, fighting at the
Battle of Seven Pines on May 31. Partially recovered, Campbell was promoted to brigadier general on March 13, 1863, and given command of the Union District of
Wisconsin in the Department of the Northwest on November 7, 1864. He led this district until the end of the war, and was mustered out of the volunteer service on January 15, 1866. ==Postbellum==