thumb|The Zimmermann telegram as it was sent from [[Washington, D.C., to Ambassador Heinrich von Eckardt, the German ambassador to Mexico.]]
thumb|[[History of Mexico#Revolution of 1910–1920|Mexico in 1916 (in dark green); territory promised to Mexico in the Zimmermann telegram (in light green); and the pre-1836 Mexican territory (red line)]]
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.
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thumb|upright|[[Arthur Zimmermann]]
The message came in the form of a coded telegram dispatched by Arthur Zimmermann, the (a top-level civil servant, second only to their respective minister) in the Foreign Office 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:
Original (German):
Translated: