Second in command of submarine in September 1917, he participated in campaigns in the Adriatic until the end of the war. In January 1919, he was sent to Rijeka to serve in the intelligence service, then in Egypt and Lebanon. Lieutenant de Vaisseau in June 1919, he commanded the submarine "Le Verrier" in the Mediterranean (1920–1922), and wrote studies of analysis on the role of "torpilleurs" during the Battle of Jutland and the employment of tactical submarines engaged in chases by groups in which he advocated the anticipation of ideas which were later realized by Admiral Donitz during the Second World War.