The Zimmermann Telegram (or Zimmermann Note) was a 1917 diplomatic proposal from the German Empire to Mexico to make war against the United States. 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.
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.
More information on the Wikipedia page [1]. There is various other coverage - eg [2], [3] and [4].