World War I: Japan and the U.S. Were Allies, Not Enemies

World War I was not fought between Japan and the United States. Both were members of the Allied Powers (Entente) opposing the Central Powers (Germany, Austria-Hungary, Ottoman Empire, Bulgaria).

Japan's Role in WWI

• Entered war: August 23, 1914 (honoring Anglo-Japanese Alliance).
• Actions: Seized German colonies in Pacific (Mariana, Caroline, Marshall Islands, Qingdao, China).
• Naval: Escorted Allied shipping in Mediterranean, Indian Ocean.
• Siberian Intervention: 1918–1922, deployed 70,000 troops to support White Russians against Bolsheviks—U.S. also participated (8,000 troops) but separately.

U.S.–Japan Relations During WWI

• Cooperated at Versailles (1919); both gained League of Nations mandates.
• Tensions emerged: Japan's Racial Equality Proposal rejected (U.S. opposition); Japanese expansion in China (Twenty-One Demands, 1915) alarmed U.S.
• Washington Naval Conference (1921–22) limited capital ships (5:5:3 ratio US:UK:Japan) — temporary détente.

Path to Pacific War (1941)

Post-WWI, divergent interests intensified: U.S. opposed Japanese expansion in Manchuria/China; Japan resented naval limits and U.S. immigration exclusion (1924). Economic sanctions (oil embargo, 1941) led to Pearl Harbor.

Bottom Line

In WWI, Japan and the U.S. were wartime allies with emerging strategic rivalry—not combatants. Their direct war began 23 years later.