A decoded telegram offering Mexico Texas back helped pull the US into WWI

In January 1917, German Foreign Secretary Arthur Zimmermann cabled Mexico proposing a wartime alliance, offering to help it reclaim Texas, New Mexico and Arizona if the US joined the fight against Germany. British codebreakers intercepted and decrypted the message, and once it hit American newspapers it swung public opinion behind entering the war.

— Barbara W. Tuchman, The Zimmermann Telegram — Macmillan, 1958
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