The Zimmermann Telegram

    This is an excerpt from chapter 3 of The Code Book by Simon Singh, a book which I highly recommend to anyone interested in the history of cryptography.

    Sun-Tzu, author of the Art of War, a text on military strategy dating from the fourth century BC, stated that: 'Nothing should be as favourably regarded as intelligence; nothing should be as generously rewarded as intelligence; nothing should be as confidential as the work of intelligence.' The French were fervent believers in the words of Sun-Tzu, and in addition to honing their cryptanalytic skills they also developed several ancillary techniques for gathering radio intelligence, methods that did not involve decipherment. For example, the French listening posts learnt to recognise a radio operator's fist. Once encrypted, a message is sent in Morse code, as a series of dots and dashes, and each operator can be identified by his pauses, the speed of transmission, and the relative lengths of dots and dashes. A fist is the equivalent of a recognisable style of handwriting. As well as operating listening posts, the French established six direction-finding stations which were able to detect where each message was coming from. Each station moved its antenna until the incoming signal was strongest, which identified a direction for the source of a message. By combining the directional information from two or more stations it was possible to locate the exact source of the enemy transmission. By combining fist information with direction-finding, it was possible to establish both the identity and the location of, say, a particular battalion. French intelligence could then trade its path over the course of several days, and potentially deduce its destination and - objective. This form of intelligence gathering, known as traffic analysis, was particularly valuable after the introduction of a new cipher. Each new cipher would make cryptanalysts temporarily impotent, but even if a message was indecipherable it could still yield information via traffic analysis.
    The vigilance of the French was in sharp contrast to the attitude of the Germans, who entered the war with no military cryptanalytic bureau. Not until 1916 did they set up the Abhorchdienst, an organisation devoted to intercepting Allied messages. Part of the reason for their tardiness in establishing the Abhorchdienst was that the German army had advanced into French territory in the early phase of the war. The French, as they retreated, destroyed the landlines, forcing the advancing Germans to rely on radios for communication. While this gave the French a continuous supply of German intercepts, the opposite was not true. As the French , were retreating back into their own territory, they still had access to their own landlines, and had no need to communicate by radio. With a lack of French radio communication, the Germans could not make many interceptions, and hence they did not bother to develop their cryptanalytic department until two years into the war.
    The British and the Americans also made important contributions to Allied cryptanalysis. The supremacy of the Allied codebreakers and their influence on the Great War are best illustrated by the decipherment of a German telegram that was intercepted by the British on 17 January 1917. The story of this decipherment shows how cryptanalysis can affect the course of war at the very highest level, and demonstrates the potentially devastating repercussions of employing inadequate encryption. Within a matter of weeks, the deciphered telegram would force America to rethink its policy of neutrality thereby shifting the balance of the war.
    Despite calls from politicians in Britain and America, President Woodrow Wilson had spent the first two years of the war steadfastly refusing to send American troops to support the Allies. Besides not wanting to sacrifice his nation's youth on the bloody battlefields of Europe, he was convinced that the war could be ended only by a negotiated settlement, and he believed that he could best serve the world if he remained neutral and acted as a mediator. In November 1916, Wilson saw hope for a settlement when Germany appointed a new Foreign Minister, Arthur Zimmermann, a jovial giant of a man who appeared to herald a new era of enlightened German diplomacy. American newspapers ran headlines such as OUR FRIEND ZIMMERMANN and LIBERALIZATION OF GERMANY, and one article proclaimed him as 'one of the most auspicious omens for the future of German-American relations'. However, unknown to the Americans, Zimmermann had no intention of pursuing peace. Instead, he was plotting to extend Germany's military aggression.
    Back in 1915, a submerged German U-boat had been responsible for sinking the ocean liner Lusitania, drowning 1,198 passengers, including 128 US civilians. The loss of the Lusitania would have drawn America into the war, were it not for Germany's reassurances that henceforth U-boats would surface before attacking, a restriction that was intended to avoid accidental attacks on civilian ships. However, on 9 January 1917, Zimmermann attended a momentous meeting at the German castle of Pless, where the Supreme High Command was trying to persuade the Kaiser that it was time to renege on their promise, and embark on a course of unrestricted submarine warfare. German commanders knew that their U-boats were almost invulnerable if they launched their torpedoes while remaining submerged, and they believed that this would prove to be the decisive factor in determining the outcome of the war. Germany had been constructing a fleet of two hundred U-boats, and the Supreme High Command argued that unrestricted U-boat aggression would cut off Britain's supply lines and starve it into submission within six months.
    A swift victory was essential. Unrestricted submarine warfare and the inevitable sinking of US civilian ships would almost certainly provoke America into declaring war on Germany. Bearing this in mind, Germany needed to force an Allied surrender before America could mobilise its troops and make an impact in the European arena. By the end of the meeting at Pless, the Kaiser was convinced that a swift victory could be achieved, and he signed an order to proceed with unrestricted U-boat warfare, which would take effect on 1 February.
    In the three weeks that remained, Zimmermann devised an insurance policy. If unrestricted U-boat warfare increased the likelihood of America entering the war, then Zimmermann had a plan that would delay and weaken American involvement in Europe, and which might even discourage it completely. Zimmermann's idea was to propose an alliance with Mexico, and persuade the president of Mexico to invade America and reclaim territories such as Texas, New Mexico and Arizona. Germany would support Mexico in its battle with their common enemy, aiding it financially and militarily.
    Furthermore, Zimmermann wanted the Mexican president to act as a mediator and persuade Japan that it too should attack America. This way, Germany would pose a threat to America's east coast, Japan would attack from the west, while Mexico invaded from the south. Zimmermann's main motive was to pose America such problems at home that it could not afford to send troops to Europe. Thus Germany could win the battle at sea, win the war in Europe and then withdraw from the American campaign. On 16 January, Zimmermann encapsulated his proposal in a telegram to the German Ambassador in Washington, who would then retransmit it to the German Ambassador in Mexico, who would finally deliver it to the Mexican President. Figure 28 shows the encrypted telegraph; the actual message is as follows: