De Wert enlisted in the United States Navy in December 1948. Following recruit training and Hospital Corps training at Naval Station Great Lakes, Illinois, he was assigned to the Naval Hospital at Portsmouth, Virginia, during 1949–1950. In July 1950, he joined the Fleet Marine Force and soon sailed for the Far East to take part in the Korean War. Landing with the 1st Marine Division at Inchon in September 1950, Hospitalman De Wert participated in operations to liberate the city of Seoul. During the rest of 1950, he was involved in the landing at Wonsan, the Chosin Reservoir Campaign and the Hungnam evacuation.