The Zimmermann Telegram was the proximate cause of American entry to World War I. The telegram was sent in 1917 as a secret communique between Arthur Zimmermann, the German Foreign Minister, and the German embassy in Mexico directing the ambassador to propose an alliance against the United States.
The Zimmermann Telegram was intercepted by British cryptographers who passed the document along to the American government. The terms of the proposed Germany-Mexico alliance were inflammatory; Germany proposed a joint declaration of war against the United States and an award of American territory to Mexico at the conclusion of hostilities. Accusations that the telegram was a British fabrication meant to draw the United States into war were put to rest a few days after its release when Arthur Zimmermann took responsibility for the proposal and acknowledged the document's legitimacy. In fact, the document had been sent over the U.S. diplomatic cable, which ran through a British relay station near Land's End. The British Foreign Office provided American authorities with the decoded document, the original telegram and the key to breaking the German cipher for independent verification. As a result of these revelations, and the resumption of Germany's unrestricted submarine warfare in the Atlantic, the United States declared war against the Central Powers.