Late in 1916 the Germans had planned to engage in unrestricted submarine warfare in an effort to cut most sea borne supplies to Great Britain and thereby ensure her defeat.
However there was the real risk that this plan would result in the United States joining the war on the side of Great Britain so Germany worked on a rather novel counter measure.
It was reasoned that if Mexico could be encouraged to join the German side, and declare war on America, then the US would be forced to commit a large slice of its military effort to defending its southern border, thereby weakening any European involvement.
Accordingly, on January 19th 1917, a diplomatic telegram in coded form was sent via Western Union from the German Foreign Secretary Arthur Zimmerman, to the German Ambassador in Mexico, Heinrich von Eckardt, offering Mexico a deal. The telegram as despatched consisted of irregular groups of 3, 4 and 5 numbers and used the top secret German cipher “13040”, that unbeknown to the Germans had been cracked by the British.
The telegram was secretly intercepted by the British and sent to “Room 40”, their top-secret code breaker group in the Admiralty. Here it was brought to the attention of a shy and small man called Nigel de Grey, who was also one of the most brilliant code-breakers in the world at that time. De Grey was a quiet and self-effacing man nicknamed “The Door-mouse” by his colleagues. Working with two other rather unlikely men, Dilly Knox and the Reverend William Montgomery, a Presbyterian Minister, the trio succeeded in decoding the telegram. The result was political dynamite. It read:
The British presented this result to Edward Bell, Secretary of the US Embassy in Britain. Bell was outraged and angered, and soon after, the situation was reported to President Wilson, with the Americans regarding the incident as growing proof of German hostility.
This event, together with the sinking of the Lusitania in March 1915, resulting in the loss of many American lives, undoubtedly played a significant role in finally bringing the United States into Word War One in April 1917. This, in turn, virtually guaranteed the eventual defeat of Germany.
De Grey helped begin the great tradition of British code-breakers carried on with such distinction by Alan Turing during World War 2 some 25 years later. “Room 40” would evolve into the famous Bletchley Park in which de Grey was also to serve.
Nigel de Grey - The "Door-mouse"
For more inside information on key events of World War One see