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.