On 5 August 1914, he joined the 60th Line Infantry Regiment (French: 60e régiment d'infanterie de ligne, 60e RIL) and ended the war with the rank of captain.

Returning from the Far East, in January 1938 he assumed command of the instruction battalion at Saïda, while he was promoted to the rank of lieutenant-colonel on 25 June 1938, prior to embarking to Morocco with the 4th Foreign Infantry Regiment 4e REI.

On 13 May 1940, at Bjervik, Norway, the 13th DBLE saw action for the first time in the Norwegian Campaign, conquering four objectives and forcing German forces to retreat while abandoning numerous prisoners, automatic arms, equipment and 10 Junkers Ju 52 transport aircraft stuck on the ice of a lake.

Barely back to France with 500 of his men, he joined the Free French Forces in England on 21 June 1940 and adopted the name de Monclar (from the village name of Monclar-de-Quercy, Tarn-et-Garonne).

In June 1941, he refused to take part in combat against Vichy French forces in the Levant and particularly the Legionnaires of the 6th Foreign Infantry Regiment 6e REI.

Following his refusal, he was replaced as commander of the 13th DBLE by Lieutenant-colonel Prince Amilakvari, who led the unit across northern Libya and into Tunisia.

[5] Over the following two years, he constantly visited the various continents where the Legion was stationed and engaged in combat, including in Algeria, Morocco, Madagascar and Indochina.

Isolated far in advance of the general battle line, completely surrounded in near-zero weather, they repelled repeated assaults by day and night by vastly superior numbers of Chinese.

I want to say that these American fighting men, with their French comrades-in-arms, measured up in every way to the battle conduct of the finest troops America and France have produced throughout their national existence.