At the outbreak of the American Civil War, Dennison's father was serving as the governor of Ohio (1860–1862) and likely influenced Dennison's direct commission into the U.S. Regular Army on August 5, 1861. He was assigned to the
2nd U.S. Artillery, a regiment whose officer corps had produced Chiefs of Artillery for the
Union Army of the Potomac and formed the backbone of the famed
U.S. Horse Artillery Brigade. Promoted to
first lieutenant on November 12, 1861, Dennison served with the 2nd U.S. Artillery throughout the Civil War, almost entirely with the Horse Artillery Brigade. He gained most of his experience while serving as a section chief with
Battery A, 2nd U.S. Artillery under the command of Captain
John C. Tidball. During the
Peninsula campaign of 1862, Dennison commanded the rear (left) section of Tidball's "flying battery." He was awarded two
brevet promotions for gallantry in combat, including the ranks of captain (for actions at
Gaines' Mill and
Malvern Hill) and major (for actions at
Antietam). In 1864, Dennison commanded
Battery G, 2nd U.S., and eventually returned to command Battery A, 2nd U.S. until the end of the war. At war's end, he received a brevet promotion to
lieutenant colonel for his overall service and conduct during the war. ==Postwar career==