Starting almost from scratch, Britain built up a considerable Signals Intelligence effort during World War I. Alistair Denniston, who was to become the post-war Director of Government Code and Cypher School (GC&CS), later wrote that in 1914 "Cryptographers did not exist as far as we knew".
A number of radio intercept stations were created, and an increasing number of cryptanalysts, linguists and radio traffic analysts enjoyed considerable success not only in decrypting messages sent by Germany and its allies, but in disseminating the intelligence produced in a timely to where it was. needed, and integrating it with other sources of information.
The most famous Sigint report of WWI came from the decryption of a telegram sent by the German Foreign Minister, Count Zimmermann, in early 1917. It stated that as Germany was to undertake unrestricted submarine warfare against vessels (including neutral American ones) trading with the British, which might bring the USA into the War on the British side, Mexico would be rewarded with the recovery of its territories of Arizona, New Mexico and Texas if it joined the conflict. Release of the Zimmerman Telegram to the US authorities was one of the deciding factors in the USA joining the War on the side of the Allies.
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