Holt was 20 years old when he sailed for France in 1917 to join the American Ambulance Field Service. His patriotism, idealism, and desire for adventure had motivated him to volunteer just before the U.S. declared war on Germany. When his six-month contract was completed, he enlisted in the American Air Service where he won his pilot's wings, then applied for observer training. Holt was assigned as an observer-bombardier-gunner with the famed 20th Aero Squadron of the American 1st Daylight Bombardment Group, which dropped more bombs than any other American squadron at the front in World War I. He served near Verdun during the French summer offensive of 1917 where he was gassed.