empire included Taiwan, Korea, the Ryukyu Islands, the southern half of Sakhalin island (Karafuto Prefecture), the Kuril Islands, and Port Arthur (Kwantung Leased Territory). The policy of Nanshin-ron ("Southern Expansion Doctrine"), popular with the IJN, held that Southeast Asia and the Pacific Islands were the area of greatest potential value to the Japanese Empire for economic and territorial expansion. The Anglo-Japanese Alliance of 1902 had been signed primarily to serve Britain's and Japan's common interest of opposing Russian expansion. Amongst other provisions the treaty called on each party to support the other in a war against more than one power, although it did not require a signatory state to go to war to aid the other. Within hours of Britain's declaration of war on Germany in 1914, Japan invoked the treaty and offered to declare war on the German Empire if it could take German territories in China and the South Pacific. The British government officially asked Japan for assistance in destroying the raiders from the Imperial German Navy in and around Chinese waters, and Japan sent Germany an ultimatum demanding that it vacate China and the Marshall, Marianas and Caroline Islands. The ultimatum went unanswered and Japan formally declared war on Germany on 23 August 1914. Japan participated in a joint operation with British forces in autumn 1914 in the Siege of Tsingtao (Qingdao) to capture the Kiautschou Bay concession in China's Shandong Province. The Japanese Navy was tasked with pursuing and destroying the German East Asiatic Squadron and protection of the shipping lanes for Allied commerce in the Pacific and Indian Oceans. During the course of this operation, the Japanese Navy seized the German possessions in the Marianas, Carolines, Marshall Islands and Palau groups by October 1914. After the end of World War I, the protectorate of German New Guinea was divided amongst the war's victors by the Treaty of Versailles. The southern part of the protectorate was mandated to come under Australian administration as the Territory of New Guinea, consisting of Kaiser-Wilhelmsland (the German territory on the island of New Guinea) and the German-controlled islands south of the equator. Meanwhile, Japanese occupation of the northern part of the protectorate, consisting of the Micronesian islands north of the equator, was formally recognised by the treaty. Japan was given a League of Nations Class C mandate to govern them, the C Class being assigned because the Mandates Commission regarded