Mordacq's early military years as lieutenant were in Algeria before joining the First Regiment of the Foreign Legion in French Indochina in 1893. Fighting off the remaining pirates in Tonkin, he became an intelligence officer during the Colonnes du Nord in 1896 where he met Galliéni and Lyautey. He came back to Algeria for a year and then studied in Paris at the Ecole de Guerre to become a staff officer. He then became known for his military writing pushing for reforms from the military schools to tactical warfare. He became chief of staff of Georges Picquart's 10th Division of Infantry. When Picquart became Secretary of War in Clemenceau's first Government in 1906, he became very influential as Picquart's right hand. He pushed for the nomination of former mentor Ferdinand Foch at the head