During World War I, British authorities give Walter H. Page, the U.S. ambassador to Britain, a copy of the “Zimmermann Note,” a coded message from Arthur Zimmermann, the German foreign secretary, to Count Johann von Bernstorff, the German ambassador to Mexico. In the telegram, intercepted and deciphered by British intelligence in late January, Zimmermann stated that in the event of war with the United States, Mexico should be asked to enter the conflict as a German ally. In return, Germany promised to restore to Mexico the lost territories of Texas, New Mexico, and Arizona.
After receiving the telegram, Page promptly sent a copy to U.S. President Woodrow Wilson, who in early March allowed the U.S. State Department to publish the note. The press initially treated the telegram as a hoax, but Arthur Zimmermann himself confirmed its authenticity. The Zimmermann Note helped turn U.S. public opinion, already severely strained by repeated German attacks on U.S. ships, firmly against Germany. On April 2, President Wilson, who had initially sought a peaceful resolution to end World War I, urged the immediate U.S. entrance into the war. Four days later, Congress formally declared war against Germany.