He served in the
Union army during the
Civil War. Shortly after the outbreak of the war he was commissioned as a 2nd lieutenant in the 2nd Cavalry Regiment on June 11, 1861. He was later transferred to the 5th Cavalry on August 3, 1861. For much of the war, he served on the staff of his father, General
Edwin V. Sumner. He participated in the
Battle of Fair Oaks, Virginia, on June 1, 1862. He was promoted to 1st lieutenant on July 17, 1862, and to aide-de-camp with the rank of captain on August 20 of the same year. He fought at the
Battle of Antietam on September 17, 1862. On March 30, 1864, he was assigned as a captain in the 5th Cavalry. During the war, he received brevets (honorary promotions) to 1st lieutenant, captain, and major for "gallant and meritorious service". ==Post Civil War==