[1][3] In April 1944, with 120 other agents as part of Operation Sussex, he was tasked with gathering intelligence about enemy military installations, supply depots and troop movements in northern France, in preparation for the upcoming Allied invasion.

Deployed by parachute near Chartres, disguised as a postal worker, Veuve acquired and transmitted information about crucial enemy infrastructure, such as Le Bourget airport, an oil refinery and an underground rocket factory.

Although no provision had been made for the transmission of intelligence by courier, he showed great initiative in getting through to London two pouches of very valuable information on a naval powder factory and an oil refinery, both of which were subsequently heavily bombarded by allied aircraft.

After the Allied invasion, Joyeuse shifted his operations further inland, on one occasion narrowly escaping an SS raid with a bullet wound in his foot, while his two bodyguards (Louis Barrault and Pierre Gastaud)[5] were captured and executed on 18 August 1944 in Aulnay Sous Bois.

The appalling death rate – he estimated that only one in 12 wounded survived – inspired him to help find better treatments for trauma victims, and in 1950 he gained admission to the medical school at the University of Paris.

After graduation, the couple emigrated to the United States, where Joyeuse worked as an emergency and trauma surgeon at the Mayo Clinic while also pursuing a master's degree in surgery at the University of Minnesota, and later as a researcher at the UCLA medical school.