What event triggered the U.S. entry into World War I?

The U.S. entry into World War I was significantly influenced by the Zimmerman Telegram, which was a secret diplomatic communication sent by the German Empire to Mexico in January 1917. In the telegram, Germany proposed a military alliance with Mexico in the event the United States entered the war against Germany. Moreover, it promised Mexico the territories of Texas, New Mexico, and Arizona if the Central Powers were victorious.

This message was intercepted and decoded by British intelligence and subsequently made public, leading to widespread outrage among the American public and further solidifying anti-German sentiment in the United States. The prospect of a German alliance with Mexico, coupled with ongoing unrestricted submarine warfare by Germany that threatened American lives and interests, created a strong impetus for the U.S. to abandon its neutral stance and enter the war on the side of the Allies in April 1917.

While the sinking of the Lusitania and the assassination of Archduke Franz Ferdinand were significant incidents related to the broader context of the war, they were not the direct triggers for American involvement. The attack on Pearl Harbor is unrelated to World War I and pertains to World War II.