Question: when did the usa join world war one?
Answer: April 1917, April 6 , 1917, in April 1917

In 1917 , with Russia experiencing political upheaval following widespread disillusionment there over the war , and with Britain and France low on credit , Germany appeared to have the upper hand in Europe , while Germany 's ally , the Ottoman Empire , clung stubbornly to its possessions in the Middle East . In the same year , Germany decided to resume unrestricted submarine warfare against any vessel approaching British waters ; this attempt to starve Britain into surrender was balanced against the knowledge that it would almost certainly bring the United States into the war . Germany also made a secret offer to help Mexico regain territories lost in the Mexican -- American War in an encoded telegram known as the Zimmermann Telegram , which was intercepted by British Intelligence . Publication of that communique outraged Americans just as German U-boats started sinking American merchant ships in the North Atlantic . Wilson then asked Congress for '' a war to end all wars '' that would '' make the world safe for democracy '' , and Congress voted to declare war on Germany on April 6 , 1917 .