[4] Subsequently, he become a member of the Scottish Rite Serenenissima Gran Loggia d'Italia located in Rome, where on 15 August 1918 he received the 33rd and highest degree.

In this capacity, Cavallero was instrumental in forming plans that led to Italian victories at Piave and Vittorio Veneto during World War I.

After Italy entered World War II, on 6 December 1940 Cavallero replaced Pietro Badoglio as Chief of the Defence Staff.

The Stato Maggiore Generale was redesignated Comando Supremo, made more efficient and transformed from a mere advisory body into a true military high command.

Cavallero worked closely with German Field Marshal Albert Kesselring; however he had a rather conflicting relationship with Field Marshal Erwin Rommel, whose advance into Egypt after his success at the Battle of Gazala he opposed, advocating instead the planned invasion of Malta; his opinion, shared by Kesselring and Rintelen, was however discounted by Hitler and Mussolini.

[2] In January 1943, after the definitive loss of the African campaign and the setbacks suffered by the Italian Army in Russia, Cavallero was dismissed and replaced by General Vittorio Ambrosio.

After Mussolini's government was toppled by the King, the newly appointed Prime Minister Pietro Badoglio ordered the arrest of Cavallero.

Kesselring offered Cavallero command of the forming armed forces of the Italian Social Republic, but the discovery of the letter led some to question his loyalty.