After the
American Civil War started in early 1861, the New York Militia's
20th Infantry Regiment was among the units mobilized for three months' service; Van Buren joined as regimental engineer officer with the rank of
captain, and served in and around
Annapolis Maryland until the regiment's demobilization that Summer. Van Buren remained on active duty after the 20th Regiment completed its initial period of service, and in August 1861 he joined the staff of
John Adams Dix, then a
major general, who commanded the Departments of Maryland and Pennsylvania. Initially assigned as Dix's adjutant and chief of staff with the rank of
major, Van Buren was promoted to
lieutenant colonel later that month; in June 1862, he was promoted to
colonel an assigned as Dix's
aide-de-camp. Van Buren remained with Dix as Dix successively commanded a division of the
Army of the Potomac, the Middle District (headquartered in
Baltimore), the
VII Army Corps, and the
Department of the East. He later served as assistant adjutant of the Department of the East under
Joseph Hooker, and in March 1865 he was promoted to
brigadier general by
brevet in recognition of his superior service during the war. After the war, Van Buren was active in the
Military Order of the Loyal Legion of the United States. ==Later career==