Japan during World War I

Japan participated in World War I from 1914 to 1918 in an alliance with Entente Powers and played an important role in securing the sea lanes in the West Pacific and Indian Oceans against the Imperial German Navy. Politically, Japan seized the opportunity to expand its sphere of influence in China, and to gain recognition as a great power in postwar geopolitics.

Japan's military, taking advantage of the great distances and Germany's preoccupation with the war in Europe, seized German possessions in the Pacific and East Asia, but there was no large-scale mobilization of the economy. Foreign Minister Katō Takaaki and Prime Minister Ōkuma Shigenobu wanted to use the opportunity to expand Japanese influence in China. They enlisted Sun Yat-sen (1866–1925), then in exile in Japan, but they had little success. The Imperial Japanese Navy, a nearly autonomous bureaucratic institution, made its own decision to undertake expansion in the Pacific. It captured Germany's Micronesian territories north of the equator, and ruled the islands until 1921. The operation gave the Navy a rationale for enlarging its budget to double the Army budget and expanding the fleet. The Navy thus gained significant political influence over national and international affairs.

In the first week of World War I Japan proposed to the United Kingdom, its ally since 1902, that Japan would enter the war if it could take Germany's Pacific territories. On August 7th 1914, the British government officially asked Japan for assistance in destroying the raiders from the Imperial German Navy in and around Chinese waters. Japan sent Germany an ultimatum on August 14th 1914, which went unanswered; Japan then formally declared war on Germany on August 23rd, 1914. As Vienna refused to withdraw the Austro-Hungarian cruiser SMS Kaiserin Elisabeth from Tsingtao, Japan declared war on Austria-Hungary, too, on August 25th 1914.

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