The event that led to the United States joining the First World War was the Zimmerman Telegram. The Zimmerman Telegram was a secret diplomatic communication sent by the German Foreign Secretary, Arthur Zimmerman, to the German ambassador in Mexico in January 1917. The telegram proposed a military alliance between Germany and Mexico in the event that the United States entered the war against Germany.

Background:
- Prior to the Zimmerman Telegram, the United States had maintained a policy of neutrality in the war.
- However, Germany's unrestricted submarine warfare campaign had led to the sinking of several American ships, including the RMS Lusitania in 1915, which resulted in the deaths of over 100 American citizens.
- These incidents had already strained relations between the United States and Germany.

The Zimmerman Telegram:
- In an attempt to prevent the United States from entering the war, Germany sent the Zimmerman Telegram to Mexico.
- The telegram proposed that if Mexico were to join the war on the side of Germany, they would be rewarded with financial and military support, as well as the promise of regaining territories in the southwestern United States that had been lost in the Mexican-American War.
- The telegram also suggested that Mexico should encourage Japan to join the war against the United States.

Revelation and Impact:
- The Zimmerman Telegram was intercepted and deciphered by British intelligence, who then passed it on to the United States.
- When the contents of the telegram were made public in March 1917, it caused outrage in the United States and significantly shifted public opinion against Germany.
- The telegram was seen as a direct threat to American national security and an act of aggression.
- This revelation, along with Germany's aggressive submarine warfare and other factors, led President Woodrow Wilson to ask Congress for a declaration of war against Germany.

Conclusion:
In summary, the Zimmerman Telegram was the event that ultimately led to the United States joining the First World War. The proposal of a military alliance between Germany and Mexico, as well as the threat it posed to American national security, prompted the United States to abandon its policy of neutrality and enter the war on the side of the Allies.