feb 1, 1917 - Zimmerman Telegram intercepted and published.

Description:

A secret letter written by Germany proposing a military alliance between Germany and Mexico is intercepted and publish by British intelligence. This letter offered Germany's support in the annexation of border sates by Mexico. This ignited outrage in the American public.
This is one of the events often claimed to be the reason for U.S involvement in the war.

Source:
Wikipedia, "Zimmerman Telegram"
(https://en.wikipedia.org/wiki/Zimmermann_Telegram)

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