On August 15, 1914, the government of Japan sends an ultimatum to Germany, demanding the removal of all German ships from Japanese and Chinese waters and the surrender of control of Tsingtao—the location of Germany’s largest overseas naval bases, located on China’s Shantung Peninsula—to Japan by noon on August 23.

The previous August 6, the day after Britain entered World War I against Germany, the British foreign secretary, Sir Edward Grey, had requested limited naval assistance from the Japanese navy in hunting down armed German merchant ships. Japan gladly agreed, seeing the war as a great opportunity to pursue its own interests in the Far East. As one Japanese statesman, Inoue Karou, put it, the war was “divine aid…for the development of the destiny of Japan.” Thus the Japanese hurried to honor their 1902 alliance agreement with Britain, serving Germany with its ultimatum on August 15.

“We consider it highly important and necessary in the present situation to take measures to remove the causes of all disturbance of peace in the Far East,” the ultimatum began, “and to safeguard general interest as contemplated in the Agreement of Alliance between Japan and Great Britain.” When Germany did not respond, Japan declared war on August 23; its navy immediately began preparing an assault against Tsingtao. With Britain contributing two battalions to Japan’s force of 60,000, the Japanese approached the naval base across China, breaching that country’s neutrality. On November 7, the German garrison at Tsingtao surrendered, and Japanese troops were home by the end of the year.

The most important initial result of Japan’s entry into World War I on the side of the Allies was to free a great number of Russian forces from having to defend against Germany from the east. For his part, Japan’s foreign minister, Kato Tataki, would skillfully use World War I to redefine his country’s relationship with its most important rival, China, and to assert its supremacy in the Far East. Forcing an internally divided China to submit to the majority of the humiliating 21 Demands in early 1915, Kato extended Japan’s control over the Shantung Peninsula and indirectly over the rest of China. The Japanese economy began to boom during wartime, largely on the strength of the exploitation of Chinese raw materials and labor. As part of the post-war settlement at Versailles, Japan was given control of the Pacific Islands formerly under German rule, and allowed to maintain its hold on Shantung, at least until Chinese sovereignty was restored in 1922.

Japan’s aggressive actions against China and quick economic expansion during World War I—while the great powers of Europe were occupied elsewhere—would have far-reaching effects over the course of the 20th century. Over the coming years, ambitious militarist leaders would assert their hold ever more strongly on the Japanese government and its powerful economy, clashing brutally with China and other rivals in the Far East while readying themselves for another great struggle many of them had long anticipated: between Japan and the United States.