The Zimmermann Telegram (or Zimmermann Note or Zimmerman Cable) was a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico if the United States entered World War I against Germany. Mexico would recover Texas, Arizona, and New Mexico. The telegram was intercepted and decoded by British intelligence.

Revelation of the contents enraged Americans, especially after German Foreign Secretary Arthur Zimmermann publicly admitted on March 3 that the telegram was genuine. It helped to generate support for the American declaration of war on Germany in April.The decryption was described as the most significant intelligence triumph for Britain during World War I, and one of the earliest occasions on which a piece of signal intelligence influenced world events.