Born in Naples(1861), 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 the 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.
The Battle of Caporetto, in October 1917, was disastrous to the army, and on November 8, 1917, he was called to succeed Cadorna as chief of general staff. Having recovered what remained of the army, he organized the resistance on Mount Grappa and at the Battle of the Piave River. In 1918 he led the Italian troops in the Battle of Vittorio Veneto, and in his famous Bollettino della Vittoria (Victory Address) he communicated the rout of the Austrian army and success of the Italians.
On November 1, 1921 Diaz was in Kansas City to take part in the groundbreaking ceremony for the Liberty Memorial that was being constructed there. Also present that day were Lieutenant General Baron Jacques of Belgium, Admiral David Beatty of Great Britain, Marshal Ferdinand Foch of France and General John J. Pershing of the United States. One of the main speakers was Vice President Calvin Coolidge of the United States. In 1935 bas-reliefs of Jacques, Foch, Diaz and Pershing by sculptor Walker Hancock were added to the memorial.
After the war Diaz became a senator. In 1921 he was ennobled by King Victor Emmanuel III and given the victory title Duca della Vittoria ("Duke of Victory"). Benito Mussolini named him Minister of War, and he was promoted to Field Marshal. Upon retirement, in 1924, he was given the honor of Marshal of Italy (Maresciallo d'Italia). He died in 1928. Diaz was buried in the church of Santa Maria degli Angeli e dei Martiri, next to admiral Paolo Thaon di Revel.