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.