The Zimmermann Telegram
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January 1917: World War I in deadlock. America stays neutral. Germans think
of using unrestricted U-boat war (up to then, they had agreed to have the Uboats surface before attack), in the hope of forcing England to surrender
quickly (cutting off the supply lines).
Arthur Zimmermann, (new) German Foreign Minister.
President Woodrow Wilson reluctant to enter the war (won re-election
campaign with slogan “He kept us out of the war” shortly before).
U-boat war might provoke the US to enter the war on the side of the Allies. So
Zimmermann tries to set up a safety net in order to keep the US busy at other
fronts: he wants to offer Mexico an alliance against the USA and (if possible)
to also include Japan. That way, the US would have to fight off aggressions in
the south and west.
Zimmermann sends a message with instructions to Germany’s Minister to
Mexico, von Eckhardt, via Washington and via Sweden and Buenos Aires.
England had cut Germany’s transatlantic cables on the first day of the war, so
Germany is forced to use lines touching England, where the messages are
intercepted. The message is therefore encrypted using a (two-part) codebook,
code 0075.
This codebook had been in use for only about half a year; but it was at least
partly known to the English specialists in “Room 40” (through studying of all
the intercepts accumulated so far).
Room 40 recognized its importance, but could not yet provide a complete
decipherment. Sir William Hall, Director of Naval Intelligence, did not yet
pass the information on, in the hope that the Americans would enter the war in
any case.
America broke off diplomatic contact with Germany on February 4, 1917,
after the Germans started unrestricted U-boat war. But the US did not enter the
war.
In order to hide England’s intercepting and code-breaking activities and also
to corroborate the decipherment, Hall obtained a copy of the telegram as it was
sent from Washington to Mexico, in a different code that had been used for
several years and had a simpler structure, so it was fairly well known to Room
40.
On February 23, 1917, the telegram in this form was given to the Americans
and published in the US press on March 1.
Mexico, Japan and German diplomats denied everything, but Zimmermann
himself admitted to it (March 29). Anyway, the British had been able to
convince the American of the telegram’s authenticity already.
About a month later, the US declared war against Germany, in the end leading
to the Allies’ victory.
Kahn: “No other single cryptanalysis has had such enormous consequences.
Never before or since has so much turned upon the solution of a secret
message. For those few moments in time, the codebreakers held history in the
palm of their hand.”