Final answer:
The Zimmermann Telegram was an attempt by Germany to forge an alliance with Mexico against the United States during World War I, promising support for Mexico to regain lost territories.
Step-by-step explanation:
The Zimmermann Telegram was a pivotal communication sent by German Foreign Minister Arthur Zimmermann during World War I. The telegram was intercepted and deciphered by British intelligence and revealed a German proposal to Mexico for a military alliance against the United States. In the event that the U.S. joined the war on the side of the Allies, Germany encouraged Mexico to invade U.S. territory with the promise of assisting Mexico to 'reconquer the lost territory in Texas, New Mexico, and Arizona.' This overt act of aggression towards the United States played a significant role in swaying American public opinion and influencing President Woodrow Wilson’s decision to enter World War I on the side of the Allies.