Zimmermann Telegram was a 1917 diplomatic proposal from the German Empire to Mexico to make war against the United States. The proposal was caught by the British before it could get to Mexico. The revelation angered the Americans and led in part to a U.S. declaration of war in April.
The message came as a coded telegram dispatched by the Foreign Secretary of the German Empire, Arthur Zimmermann, on January 16, 1917, to the German ambassador in Washington, D.C., Johann von Bernstorff, at the height of World War I. On January 19, Bernstorff, per Zimmermann's request, forwarded the telegram 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 February 1, an act which German Chancellor Theobald von Bethmann-Hollweg feared 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. He was to offer Mexico material aid in the reclamation of territory lost during the Mexican-American War (the Southeastern section of the area of the Mexican Cession of 1848) and the Gadsden Purchase, specifically the American states of Texas, New Mexico, and Arizona. Eckardt was also instructed to urge Mexico to help broker an alliance between Germany and the Japanese Empire.
The Zimmermann Telegram was intercepted and decoded by the British cryptographers of Room 40. The revelation of its contents in the American press on March 1 caused public outrage that contributed to the U.S.'s declaration of war against Germany and its allies on April 6.