After graduating, Barnum was commissioned in the Third Cavalry. In 1893, he became a distinguished graduate of the Infantry and Cavalry School and was on duty at Rock Island Arsenal from 1893 to 1894.[2]
In the Santiago campaign, he was adjutant of the Tenth Cavalry and was wounded on July 2, 1898, at San Juan Hill.[2]
From 1899 to 1902, Barnum was assistant quartermaster at the United States Military Academy. For four years, he was aide to General J. Weston in the Philippines, and from 1910 to 1911 he was adjutant of the Eighth Cavalry. In 1915, Barnum graduated from Army War College and was on General Staff duty until 1917.[2]
On October 31, 1917, he was made a brigadier general of the National Army and took command of the 183rd Infantry Brigade, 92nd Division. From December 1918 to July 1919, he was the chief of the American section of the Inter-Allied Armistice Commission.[4] From 1920 to 1923, Barnum commanded the Disciplinary Barracks at Fort Leavenworth, Kansas. On February 9, 1923, he became a brigadier general and was given command of the 18th Infantry Brigade. Barnum became a major general on June 12, 1927, and retired the following September.[2]
Barnum was commander of the Massachusetts Commander of the Loyal Legion and later served as national commander-in-chief of the organization from 1940 to 1941.[2]
^Pershing, John J., and John T. Greenwood. 2013. My life before the World War: 1860-1917; a memoir. Lexington, Kentucky: Univ. Press of Kentucky. ISBN9780813141978OCLC859376304
Marquis Who's Who, Inc. Who Was Who in American History, the Military. Chicago: Marquis Who's Who, 1975. ISBN0837932017OCLC657162692
Pershing, John J., and John T. Greenwood. 2013. My life before the World War: 1860-1917; a memoir. Lexington, Kentucky: Univ. Press of Kentucky. ISBN9780813141978OCLC859376304