Second Lieutenant Barnum was ordered to Marine Corps Schools, Quantico, Virginia, where he attended The Basic School until December 1962, when he began the Artillery Officers Orientation Course, graduating in February 1963. He was then ordered overseas and joined Battery A, 1st Battalion, 12th Marines, 3rd Marine Division on Okinawa, Japan. He served first as a forward observer and then as the battalion's liaison officer. In July 1964, he accepted appointment in the regular Marine Corps. Prior to completing his Okinawa tour, he also served as the battalion liaison officer. He was promoted to first lieutenant in December 1964.