He is known for his book Les damnés de la guerre (1934) in which he shows the misery of ordinary soldiers during World War I (1914–18) forced to fight in impossible conditions.

He met Victor Méric and joined the Ligue des Combattants de la Paix (League of Fighters for Peace) that Meric had founded in 1929.

[3] Monclin's book Les damnés de la guerre (The Wretched of the War) appeared in 1934, one of a number of pacifist publications around this time.

In August 1939, a few days before the start of World War II (1939–45) Monclin and two friends left France for Belgium, and went on to Norway and Sweden.

[5] From 1968 to 1982 Andre Arru and members of the Group Francisco Ferrer published La Libre Pensée des Bouches du Rhône (Free Thought of the Mouths of the Rhone).