Fighting off the remaining pirates in Tonkin, he became an intelligence officer during the Colonnes du Nord in 1896 where he met Joseph Gallieni and Hubert Lyautey.

He then took command of the defence of Arras, the key to the sea and again halted the German advance there, which led to the front's stabilisation and the start of trench warfare.

He was to command an infantry corps in Italy but was called in early November 1917 to become the military chief of staff in Clemenceau's second government in what was to be nicknamed the Ministère de la Victoire.

He proved himself essential to the French command's reorganisation and was Clemenceau's influential right-hand man (his main military advisor) from 1917 to 1920, thus participating extensively in the Allied victory of 1918.

He remained at his command until 1925 when he left the army, resenting the contemporary political and military leaders who alienated him for his criticism of the appeasement policy toward Germany and for his unrelenting loyalty to Clemenceau in 1920.

From 1925 to his death in 1943, Mordacq wrote more than twenty books and published dozens of articles in influential reviews to promote Clemenceau and his actions in the troubled days from November 1917 to 1920, explaining the choices and reforms which were made in order to achieve military and political victory.