During World War II, Fuller joined the United States Army. He was assigned as an infantryman to the 16th Infantry Regiment, 1st Infantry Division, and saw heavy fighting. He was involved in landings in Africa, Sicily, and Normandy, and also saw action in Belgium and Czechoslovakia. In 1945, he was present at the liberation of a German concentration camp in Falkenau and shot 16-mm footage, known as "V-E +1", that was later integrated into the French documentary "Falkenau: The Impossible" (1988). In 2014, the footage was selected for the United States National Film Registry. For his military service, Fuller was awarded the Silver Star, Bronze Star, Purple Heart, and Combat Infantryman Badge. He reached the rank of corporal. Fuller used his wartime experiences as material in his films, especially in "The Big Red One" (1980), the nickname for the 1st Infantry Division. After the war, Fuller co-authored a regimental history of the 16th Infantry.