Born in Naples to a family of Spanish heritage, Diaz began his military career as a student at the Military Academy of Turin, where he became an artillery officer. He was a colonel commanding the 93rd infantry during the Italo-Turkish War, and major general in 1914. On the outbreak of World War I, he was assigned to the high command as head of the unit's operations under General Luigi Cadorna. He was promoted to 2-star general in June, 1916, and assumed the command of the 49th division and then the 23rd army corps.