1965, he then served as Guard Officer, Marine Barracks, Naval Base Pearl Harbor, Oahu, Hawaii. From December 1965 until February 1966, Barnum served on temporary duty in Vietnam as an artillery forward observer with Company H, 2nd Battalion, 9th Marines, 3rd Marine Division. During Operation Harvest Moon, Barnum would be awarded the Medal of Honor for his actions on December 18, 1965 — for "conspicuous gallantry and intrepidity at the risk of his life above and beyond the call of duty". He had taken over command of a rifle company when its commander has been killed in action, and reorganized it for defense and successful evacuation after destroying the enemy. Barnum was promoted to captain in June 1966, after he returned to Hawaii. From March until August 1967, Barnum attended the Associate Field Artillery Officers Career Course, Fort Sill, Oklahoma. Transferred to Headquarters Marine Corps, he served as Aide-de-Camp for Lieutenant General Lewis William Walt (then the Assistant Chief of Staff for Manpower and subsequently the Assistant Commandant from January 1968 until January 1971). In October 1968, Barnum returned to South Vietnam where he served as Commanding Officer of Battery E, 2nd Battalion, 12th Marines, 3rd Marine Division. With the 3rd Marine Division redeployment from South Vietnam to Okinawa in September 1969, he remained with that unit until the following October. For his service with the battery in Vietnam, he was awarded the Bronze Star Medal with Combat "V" and Gold Star in lieu of a second award, the Navy Achievement Medal with Combat "V", the Purple Heart for wounds received, the Combat Action Ribbon, and the Vietnamese Gallantry Cross with Silver Star. 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. Barnum served as Commanding Officer of 2nd Battalion, Recruit Training Regiment, Parris Island, S.C. from 1979 to 1980. In August 1989, Barnum retired from the Marine Corps after more than 27 years of service.