The event was a crucial step toward U.S. entry into World War I.

The Zimmermann Telegram was a coded message from Zimmermann to the German ambassador to Mexico. In the telegram Zimmermann instructed his ambassador in the event of a German war with the U.S. 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 telegram was intercepted and deciphered by British intelligence, and sent to the U.S. In response, President Wilson 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.

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.