After Italy entered World War II on 10 June 1940, Campioni commanded the Italian battlefleet during the early months of the Mediterranean naval campaign against the British, including the Battle of Calabria, the Battle of Taranto, Operation White, and the Battle of Cape Spartivento. 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. He did, however, become a Commander of the Military Order of Savoy for his achievements from June 1940 through July 1941.