Japan participated in World War I from 1914-1918 in an alliance with Entente Powers. It played an important role in securing the sea lanes in the West Pacific and Indian Oceans against the Imperial German Navy as the member of the Allies.
Japan had been an ally of the United Kingdom since 1902. Japan's entry into the war in August 1914 followed Britain's request to combat raids of the German Imperial Navy in and around Chinese waters. Japan formally declared war on Germany on 23rd August and on Austria-Hungary on 25th August.