Held in high regard as the most promising officer in the Regia Marina,[1] he became commander of the 1st Naval Squadron — the Italian main battlefleet — on 15 August 1939 with the battleship Giulio Cesare as his flagship.

When Italy entered the war on the side of the Axis powers with its invasion of France on 10 June 1940, Campioni still commanded the Italian battlefleet.

Heavily criticized for failing to intercept two British convoys and handling his superior force too cautiously during the latter battle, he was relieved of command on 8 December 1940 — to be succeeded by Angelo Iachino — and returned to the post of Deputy Chief of Staff of the Navy.

In November 1941, he reached retirement age and was transferred to the naval auxiliary, although he remained on active duty as governor and commander in the Aegean.

He thereafter refused to collaborate with the Axis Powers and oversaw Italian armed resistance to the German conquest of the Aegean Islands that immediately followed the armistice.

In January 1944, they transported him to the northern portion of Italy administered by the Italian Social Republic, a German client regime led by Benito Mussolini, which took custody of him and jailed him at Verona.

Influenced by Mussolini, who wanted to point the finger at the Regia Marina as being primarily responsible for the defeat of Fascist Italy,[5] the tribunal sentenced Campioni and Mascherpi after very brief deliberations to death by firing squad.

The tribunal downgraded the charge from high treason to "harming the interests of the state," allowing the two admirals to face the more honorable death of being shot in the chest rather than in the back.