A January 1917 telegram from Arthur Zimmermann of the German Foreign Ministry to Germany's Mexico City Embassy for transmittal to the Mexican Government (a neutral) proposing that they declare war against the United States (another neutral), in return for Arizona, New Mexico and part of Texas. The British decoded it (after all, it was wide-open to anyone who knew the code), and sent to the American government.
Usage: The Zimmermann Telegram was not looked upon too benignly by America - no one really
knows why America entered WWI in April 1917, but the Zimmermann Telegram certainly didn't help. (Forget the sinking of the Lusitania
, 23 months