When German Foreign Secretary Arthur Zimmermann dispatched a coded telegram to Mexico offering American territory—Texas, New Mexico, and Arizona—in exchange for a wartime alliance, British codebreakers in Room 40 intercepted and decrypted the message almost immediately. However, exposing it presented a massive diplomatic dilemma. If the British revealed the telegram, Germany would realize their primary diplomatic code (Code 0075) was compromised, and the U.
S. might realize Britain was spying on transatlantic cables. To solve this, British Intelligence executed a masterclass in deception.
They learned that the German embassy in Washington re-transmitted the telegram to Mexico City using an older codebook (Code 13040) over commercial telegraph wires. A British agent in Mexico bribed a telegraph office employee to secure a copy of this re-transmitted version. This allowed Britain to claim the document was obtained via espionage in Mexico rather than cable-tapping, protecting Room 40's cryptographic secrecy.
Furthermore, when American newspapers published the story, critics claimed it was British propaganda—until Zimmermann himself inexplicably confessed at a press conference, confirming its authenticity. The diplomatic fallout shattered American neutrality and helped propel the United States into World War I.