Japan entered World War I in 1914, seizing the opportunity of Germany's distraction with the European War and wanting to expand its sphere of influence in China. Japan declared war on Germany in August 23, 1914 and quickly occupied German-leased territories in China's Shandong Province as well as the Marianas, Caroline, and Marshall Islands in the Pacific which were part of German New Guinea. The siege of Tsingtao, a swift invasion in the German territory of Jiaozhou (Kiautschou) proved successful and the colonial troops surrendered on 7 November 1914.

With its Western allies, notably the United Kingdom, 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 sentiment in China, and international condemnation, Japan withdrew the final group of demands, and treaties were signed in May 1915.

In 1919, Japan proposed a clause on racial equality to be included in the League of Nations covenant at the Paris Peace Conference. The clause was rejected by several Western countries and was not forwarded for larger discussion at the full meeting of the conference. The rejection was an important factor in the coming years in turning Japan away from cooperation with West and toward nationalistic policies. Anglo-Japanese Alliance was ended in 1923.