Upon his return from Okinawa, Barnum was assigned as a weapons instructor at The Basic School, Marine Corps Development and Education Command, Quantico, where he served until August 1970 at which time he entered the Amphibious Warfare School, graduating in February 1972. He served as Operations Officer, 2nd Battalion, 10th Marines, Camp Lejeune beginning in March 1972, and was promoted to major in May 1972, to lieutenant colonel in December 1978, and to colonel in February 1984. In 1987, he was assigned as Military Secretary to the 29th Commandant of the Marine Corps.