Zimmermann Telegram - British Interception

British Interception

The Telegram was sent to the German embassy in the U.S. for re-transmission to von Eckardt in Mexico. It has traditionally been claimed that the Telegram was sent by three routes: transmitted by radio, and also sent over two trans-Atlantic telegraph cables operated by neutral governments (the U.S. and Sweden) for the use of their diplomatic services. But this has now been disproved and only one method was used. The message was delivered to the US Embassy in Berlin and then transmitted by diplomatic cable to Copenhagen and then to London for onward transmission over transatlantic cable to Washington. The disinformation about the 'three routes' was spread by Reginald 'Blinker' Hall, the then head of Room 40, to try to shield from the USA the fact that Room 40 was intercepting its cable traffic.

Direct telegraph transmission was not possible because the British had cut the German cables in the Atlantic. However, the USA allowed a limited use of its diplomatic cables for Germany to communicate with its ambassador in Washington. The facility was supposed to be used for cables connected with Wilson's peace proposals.

The Swedish cable ran from Sweden; the U.S. cable from the U.S. embassy in Denmark. However, neither cable ran directly to the U.S. Both cables passed through a relay station at Porthcurno, near Land's End, the westernmost tip of England. Here the signals were boosted for the long trans-oceanic jump. All traffic through the Porthcurno relay was copied to British intelligence; in particular, to the codebreakers and analysts in Room 40 at the Admiralty. This is now known to be incorrect, though the 'Swedish Roundabout' was used for other German communications between Berlin and Washington/South & Central America, After their telegraph cables had been cut, the German Foreign Office appealed to the U.S. for use of their cable for diplomatic messages. President Wilson agreed to this, in the belief that such cooperation would sustain continued good relations with Germany, and that more efficient German-American diplomacy could assist Wilson's goal of a negotiated end to the war. The Germans handed in messages to the U.S. embassy in Berlin, which were relayed to the embassy in Denmark and then to the U.S. by American operators. However, the U.S. placed conditions on German usage: all messages must be in clear. The Germans assumed that the U.S. cable was secure, and used it extensively.

Obviously Zimmermann's note could not be given to the U.S. in clear. The Germans persuaded Ambassador James W. Gerard to accept it in coded form, and it was transmitted on 16 January 1917.

At Room 40, Nigel de Grey partially deciphered the telegram by the next day. Room 40 had previously obtained German cipher documents, including the diplomatic cipher 13040 (captured in Mesopotamia), and naval cipher 0075, retrieved from the wrecked cruiser SMS Magdeburg.

Disclosure of the Telegram would obviously sway American public opinion against Germany. But Room 40 chief "Blinker" Hall was reluctant to let it out, because the disclosure would expose Room 40's breaking of German codes, and also that Britain was eavesdropping on the U.S. cable. Hall waited three weeks. During this period, De Grey and William Montgomery completed the decryption. On 1 February Germany announced resumption of "unrestricted" submarine warfare, which led to the U.S. breaking off relations with Germany on 3 February.

Hall passed the telegram to the Foreign Office on 5 February, but still warned against releasing it. Meanwhile, the British discussed possible cover stories: to explain to the Americans how they got the ciphertext of the Telegram, without admitting to the cable snooping; and to explain how they got the cleartext of the Telegram without letting the Germans know their codes were broken.

For the first story, the British also got the ciphertext of the Telegram from the Mexican commercial telegraph office. The British knew that the German Embassy in Washington would relay the message by commercial telegraph, so the Mexican telegraph office would have the ciphertext. "Mr. H", a British agent in Mexico, bribed an employee of the commercial telegraph company for a copy of the message. (Sir Thomas Hohler, then British ambassador in Mexico, claimed to have been "Mr. H" in his autobiography.) This ciphertext could be shown to the Americans without embarrassment. The retransmission was enciphered using cipher 13040, so by mid-February the British had the complete text.

For the second story, the British claimed that their agents had stolen the Telegram's deciphered text in Mexico. The German Foreign Office refused to consider a possible code break, and instead sent von Eckardt on a witch-hunt for a traitor in the embassy in Mexico. (Von Eckardt indignantly rejected these accusations, and the Foreign Office eventually declared the embassy exonerated.)