The Zimmermann Telegram
The coded telegram in which Germany proposed a military alliance with Mexico against the United States, intercepted by British intelligence in January 1917.
On 16 January 1917, German Foreign Secretary Arthur Zimmermann sent this coded cable to Mexico proposing an alliance: if Mexico joined Germany against the United States, it would recover Texas, New Mexico, and Arizona. British codebreakers in Room 40 intercepted and deciphered the message, and its publication in American newspapers in early March 1917 inflamed public opinion at a moment when Germany had also resumed unrestricted submarine warfare. Together the two events pushed a reluctant United States into declaring war on Germany on 6 April 1917. Full story in our article on the Zimmermann Telegram.
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