Denner served as a Free French partisan in the Vercors mountains and destroyed a Nazi SS truck with a grenade; he was wounded and later received the Croix de Guerre for this operation.
Denner participated as a member of the Free French forces in the Vercors mountains, where he carried out a daring attack on a Nazi SS convoy using a grenade, resulting in his own injury. He later received the prestigious Croix de Guerre award for his bravery.