[[File:Marshal Cavallero and Rommel.jpg|thumb|left|200px|Cavallero bersama [[Erwin Rommel]].]]
Italy entered [[ World War II]], on 6 December 1940 Cavallero replaced [[Pietro Badoglio]] as [[ Chief of the Defence Staff ( Italy)|''Capo di Stato Maggiore Generale'']]; shortly after, he was sent to command the Italian forces involved in the unsuccessful [[Greco- Italian War]] until the spring of 1941. While he managed to halt the Greek advance, Cavallero was unable to break the stalemate until the German intervention. In the meantime, his role as Chief of Staff was filled by General [[Alfredo Guzzoni]].
As Chief of the Italian Supreme Command, Cavallero worked closely with [[ Germany| German]] [[Field Marshal]] [[Albert Kesselring]]; 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 [[ Operation C3| invasion of Malta]]; his opinion was however discounted. Under Cavallero’s leadership, Italy’s military forces continued to perform rather poorly during the war; nonetheless, he was promoted to [[Marshal of Italy]] (''Maresciallo d'Italia'') in 1942 after the promotion of Rommel to Field Marshal ( largely to prevent Rommel to claim rank before him). Despite having a good grasp on the problems inherent to the war in the Mediterranean that Italy had to fight, his acquiescence to Mussolini 's views ( for example his insistence on augmenting the Italian contingent fighting on the [[ Eastern Front ( World War II)| Eastern Front]]) led to a fatal dispersion of Italy's meager resources.
In January 1943, after the definitive loss of the African campaign and the setbacks suffered by the [[Armata Italiana in Russia| Italian army in Russia]], Cavallero was dismissed and replaced by General [[Vittorio Ambrosio]]. In response to Cavallero's dismissal, members of the Fascist leadership like [[Galeazzo Ciano]], openly hostile to him, openly expressed their satisfaction.
After Mussolini’s government was toppled by the [[Victor Emmanuel III|King]], the newly appointed [[ Prime Minister]] [[Pietro Badoglio]] ordered the arrest of Cavallero. In a document written in own defense, Cavallero claimed the merit of having opposed Mussolini and his regime. After [[ Italian Armistice| Italy surrendered]] in September 1943, he was freed by the Germans, and was offered by Kesselring the command of the forming armed forces of the [[ Italian Social Republic|Repubblica Sociale Italiana]], but the finding of the letter led to some suspicions.
In the morning of 14 September 1943, he was found dead by a gunshot in the garden of a hotel in [[Frascati]], after having dined and talked with Kesselring the night before; it is still up to debate wheter he committed suicide or was assassinated by the Germans. It seems however that he expressed firmly his will to refuse collaborating with the Germans.