Allen was assigned to the 3rd Battalion of the 358th Infantry Regiment, part of the 90th Division of the American Expeditionary Force (AEF) which he led into battle on the Western Front at St. Mihiel and Aincreville. During one battle Allen received a bullet through his jaw and mouth and, as a result of the wound, never stuttered again. He was awarded a Silver Star and a Purple Heart for his actions. Allen remained with the AEF in France until the Armistice of November 11, 1918. He then served with the Army of Occupation in Germany until 1920 when he returned to the United States.