In 1941 the United States entered World War II and John along with thousands of young Americans enlisted in the United States Army. He served as an artillery trajectory technician for the 30th Infantry Division. His unit took part in the D-day invasion and took part in many of the final campaigns of the European theater, including the Battle of the Bulge. His sketchbook was never far from him and he brought back a collection of landscapes and personal images of Army life, from England, Normandy, Belgium and Germany.