The Zimmermann Telegram which tells the story superbly. Giles Milton provides a more succinct accout of how a codebreaker's success and the well-timed public release of the contents brought America into that war and contributed to Allied victory:
It arrived on his desk as a jumbled series of numbers.Giles Milton: NIGEL THE DORMOUSE: THE BRILLIANT CODE-BREAKER WHO CHANGED THE COURSE OF WORLD WAR ONE
There was no pattern to the code and nor was there any obvious logic. Yet it was immediately obvious to cryptologist Nigel de Grey that he was looking at a highly secretive document. Now, his task was to decipher it with all possible speed; the British government urgently needed to know what it said.
The message had been transmitted on 16 January, 1917, by the German Foreign Secretary, Arthur Zimmermann. It contained a message intended for the president of Mexico, but it was intercepted by Room 40, Britain’s celebrated code-breaking service. .... [more]