Seizing the opportunity of Berlin's distraction with the European War and wanting to expand its sphere of influence in China, Japan declared war on Germany in August 1914 and quickly occupied German-leased territories in China's Shandong Province and the Mariana, Caroline, and Marshall islands in the Pacific. With its Western allies heavily involved in the war in Europe, Japan sought further to consolidate its position in China by presenting the Twenty-One Demands to China in January 1915. Besides expanding its control over the German holdings, Manchuria, and Inner Mongolia, Japan also sought joint ownership of a major mining and metallurgical complex in central China, prohibitions on China's ceding or leasing any coastal areas to a third power, and miscellaneous other political, economic, and military controls, which, if achieved, would have reduced China to a Japanese protectorate. In the face of slow negotiations with the Chinese government, widespread anti-Japanese sentiments in China, and international condemnation, Japan withdrew the final group of demands, and treaties were signed in May 1915.

Japan's hegemony in northern China and other parts of Asia was facilitated through other international agreements. One with Russia in 1916 helped further secure Japan's influence in Manchuria and Inner Mongolia, and agreements with France, Britain, and the United States in 1917 recognized Japan's territorial gains in China and the Pacific. The Nishihara Loans (named after Nishihara Kamezo, Tokyo's representative in Beijing) of 1917 and 1918, while aiding the Chinese government, put China still deeper into Japan's debt. Toward the end of the war, Japan increasingly filled orders for its European allies' needed war matériel, thus helping to diversify the country's industry, increase its exports, and transform Japan from a debtor to a creditor nation for the first time.

Japan's power in Asia grew with the demise of the tsarist regime in Russia and the disorder the 1917 Bolshevik Revolution left in Siberia. Wanting to seize the opportunity, the Japanese army planned to occupy Siberia as far west as Lake Baykal. To do so, Japan had to negotiate an agreement with China allowing the transit of Japanese troops through Chinese territory. Although the force was scaled back to avoid antagonizing the United States, more than 70,000 Japanese troops joined the much smaller units of the Allied Expeditionary Force sent to Siberia in 1918.

The year 1919 saw Japan sitting among the "Big Five" powers at the Versailles Peace Conference. Tokyo was granted a permanent seat on the Council of the League of Nations, and the peace treaty confirmed the transfer to Japan of Germany's rights in Shandong, a provision that led to anti-Japanese riots and a mass political movement throughout China. Similarly, Germany's former Pacific islands were put under a Japanese mandate. Despite its small role in World War I (and the Western powers' rejection of its bid for a racial equality clause in the peace treaty), Japan emerged as a major actor in international politics at the close of the war.

Data as of January 1994