At the outbreak of the First World War Estienne was made commander of the 22nd Artillery Regiment serving with Philippe Pétain's division. At the Battle of Charleroi he shocked his German opponents by the precision of his artillery fire, which was well directed due to close cooperation with aircraft. But even Estienne's competence could not save the French infantry from being destroyed by machine gun fire. In order to protect soldiers in trench warfare, he imagined mobile personnel shields to assist them.