In a crucial step toward U.S. entry into World War I, President Woodrow Wilson learns of the so-called Zimmermann Telegram, a message from German Foreign Secretary Arthur Zimmermann to the German ambassador to Mexico proposing a Mexican-German alliance in the event of a war between the U.S. and Germany.
On February 24, 1917, British authorities gave Walter Hines Page, the U.S. ambassador to Britain, a copy of the Zimmermann Telegram, a coded message from Zimmermann to Count Johann von Bernstorff, the German ambassador to Mexico. In the telegram, intercepted and deciphered by British intelligence in late January, Zimmermann instructed his ambassador, in the event of a German war with the United States, to offer significant financial aid to Mexico if it agreed to enter the conflict as a German ally. Germany also promised to restore to Mexico the lost territories of Texas, New Mexico and Arizona.
The State Department promptly sent a copy of the Zimmermann Telegram to President Wilson, who was shocked by the note's content and the next day proposed to Congress that the U.S. should start arming its ships against possible German attacks. Wilson also authorized the State Department to publish the telegram; it appeared on the front pages of American newspapers on March 1. Many Americans were horrified and declared the note a forgery; two days later, however, Zimmermann himself announced that it was genuine.
The Zimmermann Telegram helped turn the U.S. public, already angered by repeated German attacks on U.S. ships, firmly against Germany. On April 2, President Wilson, who had initially sought a peaceful resolution to World War I, urged immediate U.S. entrance into the war. Four days later, Congress formally declared war against Germany.
Taken from: http://www.history.com/this-day-in-history/president-wilson-learns-of-zimmermann-telegram [26.02.2015]