In January 1917 German foreign minister Arthur Zimmermann sent a secret telegram to his ambassador in Washington, D.C., Count Johann von Bernstorff, by three different routes, all encrypted in the same code. One was transmitted by radio from Nauen to Sayville, on Long Island; the second went via the Swedish transatlantic cable from Stockholm; and the third was delivered to the U.S. embassy in Berlin for transmission on the American cable via Copenhagen. The text announced an intention to engage in unrestricted U-boat warfare beginning 1 February and directed the ambassador to approach the Mexican government with an
offer of support if it attacked the United States to recover “lost territory in Texas, New Mexico, and Arizona.”
The intercepted text was decrypted in Room 40 at the Admiralty by the Reverend William Montgomery and Nigel de Grey and was passed to the U.S. embassy before being made public in March 1917. When challenged, Count Bernstorff confirmed the authenticity of the telegram, and as a direct consequence President Woodrow Wilson told Congress in April 1917 that America’s neutrality would cease.