What sparked the entry of the U.S. into World War I?

The entry of the United States into World War I was primarily triggered by the Zimmermann Telegram and the practice of unrestricted submarine warfare by Germany.

The Zimmermann Telegram was a secret diplomatic communication sent by the German Empire to Mexico in January 1917, proposing a military alliance against the United States. In return for Mexico’s support, Germany promised to help Mexico reclaim territories it had lost to the U.S., such as Texas, New Mexico, and Arizona. When this telegram was intercepted and made public, it caused outrage among the American public and intensified anti-German sentiment.

Additionally, Germany's policy of unrestricted submarine warfare, which involved attacking ships without warning, threatened American merchant ships and lives. The sinking of the Lusitania, a British passenger liner carrying American citizens, in 1915 had already raised alarms about German aggression at sea. Nonetheless, it was the combination of the Zimmermann Telegram and the renewed campaign of unrestricted submarine warfare in early 1917, which threatened American shipping and lives on a larger scale, that ultimately compelled the U.S. to enter the war in April 1917. This marked a significant turning point, as it brought fresh American troops and resources to the conflict, significantly impacting the war’s outcome.