The British government, which wanted to expose the incriminating telegram, faced a dilemma: If it exposed the actual telegram, the Germans would be led to suspect that their code was broken, and if it did not, it would lose an important opportunity to draw the United States into World War I. The British knew that there existed a Mexican decrypted version of the telegram, and that, if they could lay their hands on it, they could pretend that the telegram was obtained as a result of espionage activity in Mexico, and not as a result of code breaking. Therefore, the British government contacted a British agent in Mexico, known only as Mr. H., who managed to get a copy of the Mexican version of the telegram.
The telegram was delivered by Admiral Hall to the British Foreign Minister, Arthur James Balfour, who in turn invited the American ambassador in Britain, Walter Page, and delivered the telegram to him on February 23rd. Two days later he relayed it to President Woodrow Wilson.
American sentiment was generally as anti-Mexico as it was anti-German. General John J. Pershing had long been chasing Pancho Villa, who had been attacking inside Texas. This was a great expense to the government, and Wilson was leaning towards discontinuing the search until after new elections were held in Mexico, a new government installed, and a new constitution in place. News of the telegram exacerbated tensions between the U.S. and Mexico, since such a treaty, if in place, would have hindered the election of a new Mexican government more friendly to American interests.
On March 1, the US Government gave the plaintext of the telegram to the press. Initially the American public believed the telegram to be a fraud designed to bring America into the war on the Allied side. This opinion was bolstered by German, Mexican and Japanese diplomats, and by the American pacifist and pro-German lobbies who all denounced the telegram as a forgery. However, in a startling move, Zimmermann confirmed its authenticity in public, two days after its publication.
Although the telegram began by stating that Germany was most interested in maintaining American neutrality, this confirmation evoked an outpouring of anti-German sentiment. Wilson responded to this manifestation of German hostility to America by asking Congress to arm American ships so that they could fend off potential German submarine attacks. One month later, on April 2, Wilson asked Congress to declare war on Germany. Four days later, on April 6, 1917, Congress complied, bringing the United States into World War I.
German submarines had previously attacked US ships near England, so the telegram was not the only cause of the war; it was, however, critical for US public opinion. It was perceived as especially perfidious that the telegram was first transferred from the US embassy in Berlin to the German embassy in Washington before being passed on to Mexico. Once the American public believed the telegram to be real, it became all but inevitable that the US would join the Great War.
The Zimmermann telegram as it was sent from the German ambassador in Washington to Mexico. Every word was encrypted into a four or five digit number, using a codebook.
The telegram as decrypted by the British Naval Intelligence codebreakers. The word Arizona was not in the German codebook and was therefore split into smaller parts.
The telegram, completely decrypted and translated.