The sinking of the Lusitania and the Zimmerman Telegram led the United States to enter World War I on the side of the Allies.
The event that led the United States to enter World War I on the side of the Allies was the sinking of the Lusitania. The sinking of this British passenger ship by a German submarine in 1915, resulting in the deaths of over a hundred Americans, sparked outrage and shifted public sentiment towards entering the war.
In addition, the Zimmerman Telegram, intercepted in January 1917, further incited American anger as it revealed Germany's plan to instigate conflict between the United States and Mexico.
President Woodrow Wilson ultimately asked Congress for a declaration of war in April 1917, succumbing to the public demand influenced by these events.