Garnier-Duplessix became a grand officer of the Legion of Honour on 22 July 1920. Garnier-Duplessix was posted to Alexandretta in Turkey during the Franco-Turkish War of 1918-21. He was appointed adjutant general and was responsible for co-ordinating three divisions of infantry, seven batteries of artillery, a cavalry regiment and two squadrons of aircraft. This force swept from east to west across Turkish Cilicia in October 1920, meeting serious resistance at Tarsus, where it destroyed Turkish fortifications to the north of the city. After the signing of the Cilicia Peace Treaty on 9 March 1921 the French Minister of War Louis Barthou wrote to Garnier-Duplessix asking him to make arrangements for the evacuation of Cilicia. Garnier-Duplessix replied with a note stating that it was impossible to evacuate the province within the requested timespan of one month due to the difficulty in establishing a sufficient police force in that period to maintain order and prevent reprisals, particularly against Christians.