British intercepted Zimmerman telegram, but idn't want to reveal that they were spying on US communications, so they created cover story saying that they got it from a commercial telegraph in Mexico city. President Wilson faced an isolationist public, and also didn't want to reveal that the British had decoded German communications. Telegram was made public, but Americans did not believe that it was real. Zimmerman accidently confirmed its validity when talking to journalists, and the US had justification for entering WW1