During World War II, Stewart was commissioned into the Army Film and Photographic Unit (AFPU) in 1940 and in 1942 led No. 2 AFPU during the Allied landings in Tunisia. The following year he edited film footage from the fighting into the documentary "Desert Victory". In 1944 he co-directed "Tunisian Victory" with Frank Capra and John Houston, although much of that film was shot in the United States. Stewart went on to lead No. 5 AFPU, covering the D-Day landings, the Battle for Caen and the Rhine Crossing.