At the start of the American Civil War he volunteered to fight with the Union army, being first an aide at the Battle of Bull Run. He was then appointed major in the 46th Illinois Infantry Regiment, after which he served in the 2nd Illinois Artillery and later became colonel of the 72nd Illinois Infantry Regiment. During the war he was involved in numerous actions with the 72nd, including the siege and surrender of Vicksburg. In 1865 he became the provost-marshal general of the Department of the Gulf and in the same year was appointed brevet brigadier general for gallant and meritorious services. He left the army and travelled to Europe, returning to help form the Grand Army of the Republic, an organization for the veterans of the American Civil War who fought for the Union. He became their first inspector general and was also involved with the design of the medal.