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 as a member of the Allies. Politically, the Japanese Empire 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 Imperial 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 area. It captured Germany's Micronesian territories north of the equator, and ruled the islands until they were transitioned to civilian control in 1921. The operation gave the Navy a rationale for enlarging its budget to double the Army budget and expanding the fleet. The Navy then gained significant political influence over national and international affairs.