In accordance with the Anglo-Japanese Alliance, the fleet helped defend Allied shipping in the Mediterranean theater of operations of World War I.

The two countries renewed the treaty once again as Britain intended to relegate the responsibility of safeguarding its shipping in the Pacific and the Mediterranean to Japan and France respectively, in the event of a war.

On 15 August, Japan issued Germany an ultimatum demanding the handover of the Kiautschou Bay concession and that German ships abandon Chinese waters, thus intervening into World War I on the side of the Triple Entente.

This proposal was refused after chief of national staff Shimamura Hayao voiced the opinion that the presence of a Japanese force at such a distance from the homeland would create a risk of an American invasion.

The above concessions in tandem with the appointment of Terauchi Masatake (who advocated wider cooperation with the British) to the post of Prime Minister of Japan, led to the creation of the 1st Special Squadron that was tasked with escorting troopships from Australia and New Zealand to the Aden Protectorate as well as patrol duties on the same route.

The 11th Flotilla departed Japan on 18 February 1917, joining the rest of the squadron in Singapore on 5 March, where Admiral Kōzō Satō assumed command.

On 4 May 1917, Sakaki and Matsu took part in the rescue of the passengers of British transport SS Transylvania, which was sinking after being torpedoed off the Gulf of Genoa, almost 3,000 people were saved.

On 27 August 1917, admiral superintendent of the Malta Dockyard George Alexander Ballard praised the operational capacity of the Japanese, favorably comparing them towards those of the French and Italians.

In December 1918, Izumo, accompanied by the destroyers Hinoki and Yanagi, sailed from Malta to Scapa Flow to assume control of seven captured German U-boats as prizes of war.