What event led to the United States entering World War I?

The interception of the Zimmermann Telegram and unrestricted submarine warfare is the event that ultimately pushed the United States to enter World War I. The Zimmermann Telegram, sent by German Foreign Minister Arthur Zimmermann to Mexico, proposed a military alliance between Germany and Mexico in the event that the U.S. entered the war against Germany. This revelation alarmed the American public and government, as it posed a direct threat to U.S. sovereignty.

Additionally, Germany's unrestricted submarine warfare, which involved attacking ships without warning, directly endangered American lives and commerce, especially with incidents such as the sinking of the Lusitania. The combination of the Zimmermann Telegram's threat and the ongoing attacks on American and allied ships created a strong impetus for the U.S. to join the Allies in the conflict. This decisive moment in history highlighted the intertwining of diplomatic communications and military actions, reshaping America's role on the world stage.