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.
The decryption has been described as the most significant intelligence triumph for Britain during World War I and it marked one of the earliest occasions on which a piece of signals intelligence influenced world events. 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 that allowed the British to decrypt the Zimmermann telegram.
The message came in the form of a coded telegram dispatched by Arthur Zimmermann, the State Secretary of Foreign Affairs of the German Empire on January 17, 1917. The message was sent to the German ambassador to Mexico, Heinrich von Eckardt. Zimmermann sent the telegram in anticipation of the resumption of unrestricted submarine warfare by Germany on February 1, which the German government presumed would almost certainly lead to war with the United States. The telegram instructed Von Eckardt that if the United States appeared certain to enter the war, he was to approach the Mexican government with a proposal for military alliance with funding from Germany. The decoded telegram was as follows:
On February 1 we intend to begin submarine
warfare without restriction. In spite of this it is our
intention to endeavour to keep the United States
neutral. If this attempt is not successful, we propose
an alliance on the following basis with Mexico:
That we shall make war together and together
make peace; we shall give ample financial support,
and it is understood that Mexico is to reconquer
her lost territory of New Mexico, Texas and
Arizona. The details are left to you for settlement.
You are instructed to inform the President of
Mexico of the above in the greatest confidence as
soon as it is certain that there will be an outbreak
of war with the United States, and suggest that the