The Zimmermann Telegram (or Zimmermann Note or Zimmerman Cable) was a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico in the prior event of the United States entering World War I against Germany. Mexico would recover Texas, Arizona, and New Mexico. The proposal was intercepted and decoded by British intelligence. Revelation of the contents enraged American public opinion, especially after the German Foreign Secretary Arthur Zimmermann publicly admitted the telegram was genuine on March 3, and helped generate support for the United States declaration of war on Germany in April. The decryption was described as the most significant intelligence triumph for Britain during World War I, and one of the earliest occasions on which a piece of signals intelligence influenced world events.
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The message came in the form of a coded telegram dispatched by Arthur Zimmermann, a Staatssekretär (i.e. a top level civil servant) in the Foreign Office of the German Empire on 19 January 1917. The message was sent to the German ambassador to Mexico, Heinrich von Eckardt. Zimmermann sent the telegram in anticipation of the resumption of unrestricted submarine warfare by Germany on 1 February, an act the German government presumed would almost certainly lead to war with the United States. The telegram instructed Ambassador Eckardt that if the United States appeared certain to enter the war, he was to approach the Mexican Government with a proposal for military alliance with funding from Germany.
The decoded telegram is as follows:
We intend to begin on the first of February unrestricted submarine warfare. We shall endeavor in spite of this to keep the United States of America neutral. In the event of this not succeeding, we make Mexico a proposal of alliance on the following basis: make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona. The settlement in detail is left to you. You will inform the President of the above most secretly as soon as the outbreak of war with the United States of America is certain and add the suggestion that he should, on his own initiative, invite Japan to immediate adherence and at the same time mediate between Japan and ourselves. Please call the President's attention to the fact that the ruthless employment of our submarines now offers the prospect of compelling England in a few months to make peace.
The Zimmermann Telegram was part of an effort carried out by the Germans to postpone the transportation of supplies and other war materials from the United States to the Allies of World War I that were at war with Germany. The main purpose of the telegram was to make the Mexican government declare war on the United States in hopes of tying down American forces and slowing the export of American arms. The German High Command believed they would be able to defeat the British and French on the Western Front and strangle Britain with unrestricted submarine warfare before American forces could be trained and shipped to Europe in sufficient numbers to aid the Allies. The Germans were encouraged by their successes on the Eastern Front into believing that they would be able to divert large numbers of troops to the Western Front in support of their goals.
Mexican President Venustiano Carranza assigned a military commission to assess the feasibility of the Mexican takeover of their former territories contemplated by Germany. The generals concluded that it would be neither possible nor even desirable to attempt such an enterprise for the following reasons:
The Carranza government was recognized de jure by the United States on 31 August 1917 as a direct consequence of the Zimmermann telegram, since recognition was necessary to ensure Mexican neutrality in World War I. After the military invasion of Veracruz in 1914, Mexico would not participate in any military excursions with the United States in World War I, thus ensuring Mexican neutrality was the best outcome that the United States could hope for, even if Mexican neutrality would allow German companies to keep their operations in Mexico open.
The telegram was sent to the German embassy in the United States for re-transmission to Eckardt in Mexico. It has traditionally been claimed that the telegram was sent over three routes: transmitted by radio and also sent over two trans-Atlantic telegraph cables operated by neutral governments (the United States and Sweden) for the use of their diplomatic services. But it has been established that only one method was used. The message was delivered to the United States Embassy in Berlin and then transmitted by diplomatic cable first to Copenhagen and then to London for onward transmission over transatlantic cable to Washington. The misinformation about the "three routes" was spread by William Reginald Hall, then the head of Room 40, to try to conceal from the United States the fact that Room 40 was intercepting its cable traffic.
Direct telegraph transmission of the telegram was not possible because the British had cut the German international cables at the outbreak of war. However, the United States allowed 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 President Woodrow Wilson's peace proposals.
The Swedish cable ran from Sweden, and the United States cable from the United States embassy in Denmark. However, neither cable ran directly to the United States. 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. After their telegraph cables had been cut, the German Foreign Office appealed to the United States 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 United States embassy in Berlin, which were relayed to the embassy in Denmark and then to the United States by American telegraph operators. However, the United States placed conditions on German usage, most notably that all messages had to be in clear (i.e., uncoded). The Germans assumed that the United States cable was secure and used it extensively.
Obviously, Zimmermann's note could not be given to the United States in clear. The Germans therefore persuaded Ambassador James W. Gerard to accept it in coded form, and it was transmitted on 16 January 1917.
In Room 40, Nigel de Grey had partially deciphered the telegram by the next day. Room 40 had previously obtained German cipher documents, including the diplomatic cipher 13040 (captured in the Mesopotamian campaign), and naval cipher 0075, retrieved from the wrecked cruiser SMS Magdeburg by the Russians, who passed it to the British.
Disclosure of the Telegram would obviously sway public opinion in the United States against Germany, provided the Americans could be convinced it was genuine. But Room 40 chief William Reginald Hall was reluctant to let it out, because the disclosure would expose the German codes broken in Room 40 and British eavesdropping on the United States cable. Hall waited three weeks. During this period, Grey and cryptographer William Montgomery completed the decryption. On 1 February Germany announced resumption of "unrestricted" submarine warfare, an act which led the United States to break off diplomatic 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 their ability to intercept American diplomatic communications (which they would continue to do for another twenty-five years); and to explain how they got the cleartext of the telegram without letting the Germans know their codes were broken. Furthermore, the British needed to find a way to convince the Americans the message was not a forgery.
For the first story, the British obtained 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", or at least involved with the interception, in his autobiography.) This ciphertext could be shown to the Americans without embarrassment. Moreover, the retransmission was enciphered using the older cipher 13040, so by mid-February the British had not only the complete text, but also the ability to release the telegram without revealing the extent to which the latest German codes had been broken—at worst, the Germans might have realized that the 13040 code had been compromised, but weighed against the possibility of United States entry into the war, that was a risk worth taking. Finally, since copies of the 13040 ciphertext would also have been deposited in the records of the American commercial telegraph, the British had the ability to prove the authenticity of the message to the United States government.
As a cover story, the British could publicly claim that their agents had stolen the telegram's deciphered text in Mexico. Privately, the British needed to give the Americans the 13040 cipher so that the United States government could verify the authenticity of the message independently with their own commercial telegraphic records; however the Americans agreed to back the official cover story. The German Foreign Office refused to consider a possible code break, and instead sent Ambassador Eckardt on a witch-hunt for a traitor in the embassy in Mexico. (Eckardt indignantly rejected these accusations, and the Foreign Office eventually declared the embassy exonerated.)
On 19 February, Hall showed the Telegram to Edward Bell, secretary of the United States Embassy in Britain. Bell was at first incredulous and thought it to be a forgery. Once Bell was convinced the message was genuine, he became enraged. On 20 February, Hall informally sent a copy to United States Ambassador Walter Hines Page. On 23 February, Page met with British Foreign Minister Arthur Balfour and was given the ciphertext, the message in German, and the English translation. Page then reported the story to President Woodrow Wilson, including details to be verified from telegraph company files in the United States. Wilson released the text to the media on 28 February 1917.
Popular sentiment in the United States at that time was anti-Mexican as well as anti-German, while in Mexico there was considerable anti-American sentiment. General John J. Pershing had long been chasing the revolutionary Pancho Villa and carried out several cross-border raids. News of the telegram further inflamed tensions between the United States and Mexico.
On the other hand, there was also a notable anti-British sentiment in the United States, particularly among German- and Irish-Americans. Many Americans wished to avoid the conflict in Europe. Since the public had been told (untruthfully) that the telegram had been stolen in a deciphered form in Mexico, the message was widely believed at first to be an elaborate forgery perpetrated by British intelligence. This belief, which was not restricted to pacifist and pro-German lobbies, was promoted by German and Mexican diplomats and by some American newspapers, especially the Hearst press empire. This presented the Wilson administration with a dilemma. With the evidence the United States had been provided confidentially by the British, Wilson realized the message was genuine—but he could not make the evidence public without compromising the British codebreaking operation.
Any doubts as to the authenticity of the telegram were removed, however, by Arthur Zimmermann himself. First at a press conference on 3 March 1917, he told an American journalist, "I cannot deny it. It is true." Then, on 29 March 1917, Zimmermann gave a speech in the Reichstag in which he admitted the telegram was genuine. Zimmermann hoped Americans would understand the idea was that Germany would only fund Mexico's war with the United States in the prior event of American entry into World War I.
On 1 February 1917, Germany began unrestricted submarine warfare against all ships in the Atlantic bearing the American flag, both passenger and merchant ships. Two ships were sunk in February, and most American shipping companies held their ships in port. Besides the highly provocative war proposal to Mexico, the telegram also mentioned "ruthless employment of our submarines." Public opinion demanded action. Wilson had previously refused to assign US Navy crews and guns to the merchant ships. However, once the Zimmermann note was public, Wilson called for arming the merchant ships, but anti-war elements in the United States Senate blocked his proposal.
Germany had long sought to incite a war between Mexico and the United States, which would have tied down American forces and slowed the export of American arms to the Allies. The Germans had engaged in a pattern of actively arming, funding and advising the Mexicans, as shown by the 1914 Ypiranga Incident and the presence of German advisors during the 1918 Battle of Ambos Nogales. The German Naval Intelligence officer Franz von Rintelen had attempted to incite a war between Mexico and the United States in 1915, giving Victoriano Huerta $12 million for that purpose. The German saboteur Lothar Witzke — responsible for the March 1917 munitions explosion at the Mare Island Naval Shipyard in the San Francisco Bay Area, and possibly responsible for the July 1916 Black Tom explosion in New Jersey — was based in Mexico City. The failure of United States troops to capture Pancho Villa in 1916 and the movement of President Carranza in favor of Germany emboldened the Germans to send the Zimmermann note.
The German provocations were partially successful. Woodrow Wilson ordered the military invasion of Veracruz in 1914 in the context of the Ypiranga Incident and against the advice of the British government. War was prevented thanks to the Niagara Falls peace conference organized by the ABC nations, but the occupation was a decisive factor in Mexican neutrality in World War I. Mexico refused to participate in the embargo against Germany and granted full guarantees to the German companies for keeping their operations open, specifically in Mexico City. These guarantees lasted for twenty-five years — coincidentally, it was on 22 May 1942 that Mexico declared war on the Axis Powers following the loss of two Mexican-flagged tankers that month to Kriegsmarine U-boats. Woodrow Wilson considered another military invasion of Veracruz and Tampico in 1917–1918, so as to take control of the Tehuantepec Isthmus and Tampico oil fields, but this time the relatively new Mexican President Venustiano Carranza threatened to destroy the oil fields in case the Marines landed there. The government of Japan, another nation mentioned in the Zimmerman Telegram, was already involved in World War I, on the side of the Allies against Germany, and later released a statement that Japan was not interested in changing sides and attacking America.
In October 2005, it was reported that an original typescript of the deciphered Zimmermann Telegram had recently been discovered by an unnamed historian who was researching and preparing an official history of the United Kingdom's Government Communications Headquarters (GCHQ). The document is believed to be the actual telegram shown to the American ambassador in London in 1917. Marked in Admiral Hall's handwriting at the top of the document are the words: "This is the one handed to Dr Page and exposed by the President." Since many of the secret documents in this incident had been destroyed, it had previously been assumed that the original typed "decrypt" was gone forever. However, after the discovery of this document, the GCHQ official historian said: "I believe that this is indeed the same document that Balfour handed to Page."
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