d'Elsa joined the Cadet Corps in Easter 1864 and on 1 April 1869 was appointed as an ensign in the 101st (2nd Royal Saxon) Grenadiers "Emperor William, King of Prussia" of the Royal Saxon Army. From October 1869 to April 1870 he was assigned to the military school in Erfurt and promoted to 2nd Lieutenant on 29 July 1870. From 1 September 1870 he was adjutant of the first battalion of his regiment, with whom he participated in the Franco-Prussian War. He participated in the battles of St. Privat, Beaumont and Sedan, and the Siege of Paris. For his service in the Franco-Prussian War, he was awarded the Iron Cross (1870).