The Zimmermann telegram (or Zimmermann note or Zimmermann cable) was a secret diplomatic communication issued from the German Foreign Office on January 17, 1917, that proposed a military contract between the German Empire and Mexico if the United States entered World War I against Germany. With Germany's aid, Mexico would recover Texas, Arizona, and New Mexico. The telegram was intercepted by British intelligence.
Revelation of the contents enraged Americans, especially after German State Secretary for Foreign Affairs Arthur Zimmermann publicly admitted on March 3, 1917, that the telegram was genuine. It helped to generate support for the American declaration of war on Germany in April 1917.[1]
The decryption has been described as the most significant intelligence triumph for Britain during World War I[2] and it marked one of the earliest occasions on which a piece of signals intelligence influenced world events.[3] The decryption was possible after the failure of the Niedermayer-Hentig Expedition to Afghanistan, when Wilhelm Wassmuss abandoned his codebook, which the Allies later recovered, and allowed the British to decrypt the Zimmermann telegram.[4]