Revelation of the contents enraged American public opinion, especially after the German Foreign Secretary Arthur Zimmermann publicly admitted the telegram was genuine on 3 March, and helped generate support for the United States declaration of war on Germany in April.
The message came in the form of a coded telegram dispatched by the Foreign Secretary of the German Empire, Arthur Zimmermann, on 19 January 1917.
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.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.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.Mexican President Venustiano Carranza assigned a military commission to assess the feasibility of the Mexican takeover of their former territories contemplated by Germany.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.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. 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.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.