How did the Zimmermann telegram influence U.S. entry into World War I?
The telegram, basically, suggested to the Mexican leaders to enter the war against the US (promising, in case of victory, Texas, New Mexico and Arizona).
The telegram was intercepted by British intelligence decrypted and later disclosed to US President Wilson.
The message could have been considered as a fake of the British to involve the US in the war but soon Zimmermann himself admitted that it was real.
The effect on the population of the United States was cataclysmic! Even the more isolationists were enraged and became convinced to enter the war against Germany.