In January 1917, Arthur Zimmermann, the State Secretary for Foreign Affairs of the German Empire, dispatched a coded telegram to the German minister in Mexico, Heinrich von Eckardt. The telegram proposed a military alliance between Germany and Mexico in the event of the United States entering World War I, offering Mexico the return of lost territory in Texas, New Mexico, and Arizona. However, the transmission path was complex.
Germany utilized American diplomatic cables under a secret arrangement, requiring the message to be routed through Washington to Mexico City. During this process, the message was decrypted from Germany's high-level Code 7500 and re-encoded using a weaker code, Code 13040, for the final leg to Mexico. British intelligence officers in Room 40 intercepted both transmissions.
Crucially, the errors and code substitutions introduced during the re-encoding step allowed British cryptographers, led by Nigel de Grey and William Montgomery, to cross-reference cipher groups and fully crack the original text without exposing that they were tapping American diplomatic traffic. When U. S.
President Woodrow Wilson released the decrypted telegram to the press, critics claimed it was a British fabrication designed to manipulate the nation. In an astonishing diplomatic blunder, Arthur Zimmermann publicly confirmed the telegram's authenticity during a press conference on March 3, 1917. This unexpected admission, paired with the cryptographic proof uncovered by Room 40, effectively erased isolationist opposition in Congress and galvanized public support, directly leading to the United States declaring war on Germany on April 6, 1917.