The United States entered World War I on April 6, 1917. While the war began in 1914, the U.S. initially maintained a position of neutrality due to public sentiment and political isolationism. However, various factors, such as unrestricted submarine warfare by Germany and the interception of the Zimmermann Telegram, shifted public opinion and government policy. American involvement brought fresh troops and resources to the Allied powers, significantly influencing the war's outcome and leading to a swift conclusion in 1918.