Zimmermann Telegram
An encrypted German telegram from January 1917 that British codebreakers read, and that helped pull the United States into the First World War.
German foreign secretary Arthur Zimmermann proposed that Mexico go to war with the US should America join the Allies, with Texas, New Mexico and Arizona as the prize. The message went out in a German diplomatic code, partly over American diplomatic cables that Germany had been allowed to borrow — which, as it happened, ran through British territory.
Britain's Room 40 intercepted and decrypted it. The awkward part was publishing it without revealing that London was reading both German and American traffic. The solution was to obtain a copy from the telegraph office in Mexico City and claim it had been stolen there. The text went public in March 1917, Zimmermann obligingly confirmed it was genuine, and in April the US declared war.