In January 1917 the United States was neutral in the European War that would eventually be called World War I. An intercepted telegram between Germany and Mexico would be the catalyst to change everything. German Foreign Minister Arthur Zimmermann sent an encoded message to the President of Mexico on January 16, 1917. British intelligence intercepted the telegram and deciphered it. After deciphering the message, the British Government passed its contents on to the American Government.
National Archives, General Records of the Department of State