At the outbreak of the First World War, Wagner abandoned his studies (which he never finished) and volunteered for the Army. He fought with Reserve Regiment 110 from 1914 to 1918 in some of the most notorious battles on the Western Front: Flanders, the Battle of Verdun, the Battle of the Somme, the Battle of Loretto, and the Battle of Champagne. He was promoted to Leutnant and decorated for bravery with the Iron Cross.