Zimmermann Telegram The Zimmermann Telegram ( or Zimmermann Note ) was a 1917 diplomatic proposal from the German Empire for Mexico to join the Central Powers , in the event of the United States entering World War I on the side of the Entente Powers . The proposal was intercepted and decoded by British intelligence . Revelation of the contents outraged American public opinion and helped generate support for the United States declaration of war on Germany in April of that year . The message came as a coded telegram dispatched by the Foreign Secretary of the German Empire , Arthur Zimmermann , on 16 January 1917 to the German ambassador in Mexico , Heinrich von Eckardt . Zimmermann sent the telegram in anticipation of the resumption of unrestricted submarine warfare by Germany on 1 February , an act which Germany predicted would draw the neutral U . S . into war on the side of the Allies . The telegram instructed Ambassador Eckardt that if the U . S . appeared likely to enter the war , he was to approach the Mexican Government with a proposal for military alliance , with funding from Germany . Mexico was promised territories in Texas , New Mexico , and Arizona that had been lost to the United States starting in 1836 as parts of the former Republic of Texas , and in 1848 with the Mexican Cession . Eckardt was also instructed to urge Mexico to help broker an alliance between Germany and the Japanese Empire . Mexico , unable to match the U . S . military , ignored the proposal and ( after the U . S . entered the war ) , officially rejected it . Contents . The Zimmermann Telegram was intercepted and decoded by the British cryptographers of Room 40 . The telegram ' s message was : Mexican response . The Zimmermann Telegram was part of an effort that was being carried out by the Germans , in order to postpone the transportation of supplies and other war materials to the Triple Entente . The Zimmermann Telegram ' s main purpose was to make the Mexican government declare a war on the U . S . , which would have tied down U . S . forces and slowed the export of U . S . arms . Mexican President Venustiano Carranza assigned a general to assess the feasibility of a Mexican takeover of their former territories . The general concluded that it would not be possible or even desirable for the following reasons : 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 President Woodrow 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 . 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 — most notably , that all messages had to be in the 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 the 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 U . S . public opinion against Germany , provided the Americans could be convinced it was genuine . 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 . Furthermore , the British needed to find a way to convince the Americans the message was not a forgery . 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 . Moreover , the retransmission was enciphered using cipher 13040 , so by mid - February the British had both the complete text and the means to prove the authenticity of the message to the U . S . government since copies of the 13040 ciphertext would also have been deposited in the records of the American commercial telegraph . For the second 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 U . S . government could independently verify the authenticity of the message 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 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 . ) British use of the Telegram . On the 19th of February , Hall showed the Telegram to Edward Bell , secretary of the U . S . Embassy in Britain . Bell was at first incredulous , thinking it was a forgery . Once Bell was convinced the message was genuine , he became enraged . On the 20th of February Hall informally sent a copy to U . S . ambassador Walter Page . On the 23rd of February , Page met with Foreign Minister Balfour , and was given the ciphertext , the message in German , and the English translation . Then Page reported the story to President Wilson , including details to be verified from telegraph company files in the U . S . Effect in the United States . Popular sentiment in the U . S . at that time was anti - Mexican as well as anti - German , while Mexico was anti - American and in some cases , anti - European . General " Black Jack " Pershing had long been chasing the revolutionary Pancho Villa , who had carried out several cross - border raids . News of the Telegram further inflamed tensions between the U . S . and Mexico . On the other hand , there was also a notable anti - British sentiment in the U . S . , particularly among German - and Irish - Americans ( the latter of whom were most recently infuriated at the British by their brutally swift suppression of the 1916 Easter Rising in Dublin ) . Until the early months of 1917 , American press coverage of Britain and France was not much more sympathetic than press coverage of Germany . Above all , the vast majority of 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 , at first the message was widely believed 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 papers , especially the Hearst press empire . This might have presented the Wilson administration with a dilemma — with the evidence the U . S . government had been confidentially provided by the British , Wilson quickly realized the message was genuine , but he could not make the evidence he had public without compromising the British codebreaking operation . However , any doubts as to the authenticity of the telegram were removed 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 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 event of American entry into World War I . On 31 January , Germany resumed " unrestricted " submarine warfare , which caused many civilian deaths , including American passengers on British ships . This brought about a wave of anti - German sentiment that spread like wildfire . The Telegram greatly increased this feeling . Besides the highly provocative anti - U . S . proposal to Mexico , the Telegram also mentioned " ruthless employment of our submarines . " It was perceived as especially offensive that the coded Telegram had been transmitted " via " the U . S . embassy in Berlin and the U . S . - operated cable from Denmark . Historical post - script . The Telegram was not an isolated case of German - Mexican collaboration , for Germany had long sought to incite a war between Mexico and the U . S . , 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 SS Ypiranga arms - shipping incident , and German advisors present 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 U . S . in 1915 , giving Victoriano Huerta $ 12 million . The German saboteur Lothar Witzke — responsible for the March 1917 munitions explosion at the Mare Island Naval Shipyard in the Bay Area , and possibly responsible for the July 1916 Black Tom explosion in New Jersey — was based in Mexico City . The failure of U . S . troops to capture Pancho Villa in 1916 , and the movement of President Carranza in favor of Germany emboldened the Germans to send the Zimmerman Note . In October 2005 , it was revealed 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 discovery of this document , the GCHQ official historian said : " I believe that this is indeed the same document that Balfour handed to Page . "