Born in Naples to a family of Italian and distant Spanish heritage, he was the son of Lodovico, a navy officer, and Irene Cecconi, the daughter of a minor noble. Diaz began his military career as a student at the Military College of Naples before moving on to the Military he moved of Turin, where he graduated as an artillery officer in 1884. Personally, Diaz was described by a contemporary journalist who saw him at 56 as "medium build, of dark complexion, with black hair turning gray and a slight cast in the eye.... His character as a soldier was that of an inflexible disciplinarian who applied to himself the same rules as he enforced on others. In the daily routine of military life, evenly poised, and in the face of danger, characteristically calm".Francis Whiting Halsey. "The Literary Digest History Of The World War Compiled From Original And Contemporary Sources". Richardson Press. Page 106.
Prewar
He was first assigned to the 10th Field Artillery Regiment. In 1890, with his promotion to captain, he was moved to the 1st Artillery. In 1894, he attended the School of War and ended the courses ranking first in his class. Then, he moved into the Army Staff and worked in the office of General Alberto Pollio for two years. In 1899, he received a promotion to infantry major and, for a year and a half, commanded a battalion of the 26th Infantry Regiment. He reached the rank of lieutenant colonel in 1905 and served as Chief of Staff in the Florence's Military Division. In 1910, as a colonel, he served in the Italo-Turkish War, commanding the 21st Infantry and, when it lost its commander, the 93rd Infantry. During his Libyan service, he was injured at Zanzur in 1912.
First World War
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. Promoted to two-star general in June 1916, he assumed the command of the 49th division and then the 23rd army corps. The Battle of Caporetto, in October 1917, was disastrous to the army, and on 8 November 1917, he was called to succeed Cadorna as chief of general staff by 9 November. Having recovered what remained of the army, he organised the resistance in 1917 on the Monte Grappa massif and along the Piave River, which successfully halted the Austro-Hungarian offensive in the First Battle of Monte Grappa. In the summer of 1918, he oversaw the victory in the Battle of the Piave River, and later that year, he led the 1.4 million Italian troops in the Battle of Vittorio Veneto, which ended the war on the Italian front. With his famous Bollettino della Vittoria, he communicated the rout of the Austro-Hungarian army and the victory of the Italians in the war.
