In the interwar period, Campioni led naval design programs at the La Spezia weapons laboratory, served as naval attaché in Paris, and held various Naval Supreme Command staff posts. In 1936, he was promoted to full admiral, and two years later, he became vice chief of staff of the Italian navy. In 1939, he was elected to the Italian Senate.
Regarded as his country's most promising naval officer, Campioni was appointed to operational command of Italy's battle fleet at the beginning of World War II. He led the fleet in a number of engagements against the British in the Mediterranean, notably the Battles of Calabria (Punta Stilo) on 9 July 1940 and Cape Teulada (Spartivento) on 27 November 1940. In the latter engagement, Campioni failed to intercept two converging British convoys. His superior naval force was hampered by the British having broken the Italian naval codes, by poor Italian reconnaissance, and by the failure of Italian air cover to materialize for well over an hour after the start of the battle. Campioni was subjected to intense criticism for his lack of aggressiveness, especially on those rare occasions when his fleet outgunned the opponent. On 8 December 1940, he was relieved of operational command and made deputy chief of staff of the Naval Supreme Command.
In July 1941, Campioni was assigned to command Axis occupation forces in the Dodecanese Islands. After Italy's surrender to the Allies on 8 September 1943, he was contacted by the British and urged to resist German attempts to establish control over the Aegean region. Although Italian forces strenuously resisted the German takeover, they were compelled to surrender on 11 September 1943.
The Germans held Campioni prisoner at Schokken (Skoki) in Poland until January 1944, when he was turned over to Benito Mussolini's puppet Italian Social Republic in northern Italy and jailed in Verona. Tried at Parma for treason, Campioni was found guilty and executed there by Fascist authorities on 24 May 1944. In November 1947, the Italian government honored Campioni by posthumously awarding him the Gold Medal for Military Valor.
John P. Vanzo and Gordon E. Hogg
Dizionario biografico degli Italiani. Vol. 17. Rome: Istituto della Enciclopedia Italiana, 1974.; Bragadin, Marc'Antonio. The Italian Navy in World War II. Menasha, WI: George Banta, 1957.; Greene, Jack, and Alessandro Massignari. The Naval War in the Mediterranean, 1940–1943. London: Chatham Publishing, 1998.; Rocca, Gianni. Fucilate gli ammiragli: La tragedia della Marina Italina nella Seconda Guerra Mondiale. Milan, Italy: Mondadori, 1990.