During the Hundred Years War, he was entrusted with defending the region between the Loire and Dordogne rivers, but he gained a dubious reputation as a bandit.
While serving under King John II at the siege of Breteuil, Arnauld led his men off the field in order to seize a castle in Normandy.
Arnaud led his company into the untouched lands of Queen of Naples Jeanne d'Anjou, taking castles and pillaging villages.
Their ultimate goal was taking Marseilles and they had pushed as far as Avignon, home of the Papacy, which caused Pope Innocent VI to open up negotiations.
He met with the Pope and his cardinals several times, confessed his sins, and was paid forty thousand crowns [20,000 gold florins] to distribute among his company, after which Arnaud led his men out of the area, giving up all the territory they had conquered.
Reconnaissance missions had led him to realize that taking Marseilles was unrealistic as the city was too populated, too well supplied and too well defended, and by April 1358, he was looking for a way to retreat.
Arnaud (as 'L'archprestre et ses Bretons') is mentioned repeatedly in Guillaume de Machaut's Le livre dou voir dit.