Schlageter was born in Schönau im Schwarzwald to Catholic parents. After the outbreak of the First World War, he became a voluntary emergency worker for the military. During the war, he participated in several battles, notably Ypres (1915), the Somme (1916) and Verdun, earning the Iron Cross both first and second class. Following his promotion to second lieutenant, he took part in the Third Battle of Ypres (1917). After the war and his dismissal from the greatly reduced army, Schlageter described himself as a student of political sciences, but he studied the subject at the most for one year.