German Foreign Minister Arthur Zimmermann sent this encoded message to the President of Mexico on January 16, 1917, offering United States territory to Mexico in return for joining the German cause. British intelligence intercepted the telegram and deciphered it. In an effort to protect their intelligence from detection and to capitalize on growing anti-German sentiment in the United States, the British waited until February 24 to present the telegram to Woodrow Wilson. The American press published news of the telegram on March 1. On April 6, 1917, the United States Congress formally declared war on Germany and its allies.
National Archives, General Records of the Department of State