The United States entered World War I in 1917. On 6 April of that year, Congress formally declared war on Germany after President Woodrow Wilson argued that German actions threatened American security and international order.
The immediate pressures included Germany's resumption of unrestricted submarine warfare, which endangered neutral and American shipping, and the Zimmermann Telegram. In that secret message, Germany encouraged Mexico to consider war against the United States in exchange for the possibility of recovering lost territory. Its publication intensified American public anger.
The United States had tried to remain neutral since the war began in 1914, although it supplied and financed the Allied powers. American troops began arriving in Europe later in 1917, and the American Expeditionary Forces played their largest role during the final offensives of 1918. A common mix-up is treating the sinking of the Lusitania in 1915 as the date of entry; it increased tensions but did not immediately bring a declaration of war. The United States declared war on Austria-Hungary separately in December 1917.