The event that prompted the United States to enter World War I in 1917 was the interception of the Zimmermann Telegram and the continued unrestricted submarine warfare by Germany. The telegram, sent by German Foreign Minister Arthur Zimmermann to Mexico, proposed a military alliance between Germany and Mexico against the United States. This revelation outraged the American public and contributed to anti-German sentiment. Additionally, Germany's resumption of submarine attacks on American ships led to significant loss of lives and property. These factors galvanized U.S. public opinion and ultimately led to President Woodrow Wilson requesting a declaration of war against Germany in April 1917.