Japan was part of the Allied Powers during World War I.

The story behind the answer

True. Japan joined the Allies in 1914 under the Anglo-Japanese Alliance (1902), declaring war on Germany. Japanese forces captured Germany’s colonies at Qingdao (Tsingtao) in China and seized German-held Pacific islands, while the Imperial Navy escorted Allied shipping in the Indian and Mediterranean seas.

Remember it this way: “WWI friends, WWII foes.” Japan fought with the Allies in World War I but later aligned with the Axis in World War II. Its WWI gains—mandates over Pacific islands and influence in Shandong—boosted its regional ambitions and stirred tensions in China (e.g., the May Fourth Movement). These outcomes shaped the path to interwar rivalries and Japan’s later militarism.