 Seward was appointed lieutenant colonel of New York's 138th Infantry Regiment, which became the 9th New York Heavy Artillery Regiment in December 1862.The regiment served in the defenses of Washington, D.C. until it was converted back to an infantry regiment and sent to the Army of the Potomac because of the losses sustained by that army in the Overland Campaign.After fighting at the Battle of Cold Harbor, Seward was appointed colonel of the regiment on June 10, 1864.