De Bono then fought in the First World War in which he distinguished himself against Austria-Hungary on the Karst Plateau in 1915 (as Colonel in the Bersaglieri corps), in the capture of Gorizia in 1916 (as commander of the "Trapani" Infantry Brigade), in the Second Battle of the Piave River in June 1918 and in the battle of Monte Grappa in October 1918 (as commander of the IX Army Corps). He was also the author of a popular patriotic song, "Monte Grappa tu sei la mia patria" ("Mount Grappa, you are my Fatherland"). During the war he was awarded three Silver Medals of Military Valour; in 1920, he was discharged with the rank of Major General.