The Zimmermann Telegram was a secret message from the German Foreign Secretary, Arthur Zimmermann, to the German Ambassador in Mexico, Heinrich von Eckardt. The telegram proposed an alliance between Germany and Mexico in the event of the United States entering World War I. In exchange for Mexico's support, Germany promised to help Mexico regain its lost territories in the American Southwest.
The Zimmermann Telegram was intercepted and decoded by British intelligence, and was eventually shared with the United States. The telegram's contents outraged American public opinion, and helped to convince President Wilson that the United States could no longer avoid involvement in World War I.