When Italy formally joined the Second World War by declaring war on France and Britain on 10 June 1940, "Fiume" was assigned to the 1st Division with "Zara" and the four destroyers of the 9th Destroyer Flotilla. The unit was assigned to the 1st Squadron, under the command of Admiral Inigo Campioni. Two days later, "Fiume" and the rest of the 1st Division, along with the 9th Division, sortied in response to British attacks on Italian positions in Libya. While they were at sea, the British submarine unsuccessfully attacked "Fiume" and her sister . On 6 July, a convoy left Naples, bound for North Africa; the following day, Italian reconnaissance reported a British cruiser squadron to have arrived in Malta. The Italian naval high command therefore ordered the 1st Division and several other cruisers and destroyers to join the escort for the convoy. The battleships "Conte di Cavour" and provide distant support. Two days later, the Italian fleet briefly clashed with the British Mediterranean Fleet in an inconclusive action off Calabria.