What event directly resulted in the United States joining World War I?

The decision for the United States to enter World War I was primarily influenced by a combination of the sinking of the Lusitania and the interception of the Zimmerman Telegram. The Lusitania, a British passenger ship, was sunk by a German U-boat in May 1915, resulting in the deaths of 128 Americans. This event stirred public outrage and heightened anti-German sentiment in the U.S.

Additionally, the Zimmerman Telegram, sent by the German government to Mexico in January 1917, proposed a military alliance between Germany and Mexico should the U.S. enter the war against Germany. The British intercepted and decoded this telegram, then shared it with the U.S. government. The revelation of the telegram provoked further anger among the American public and policy-makers, as it implied a direct threat to U.S. sovereignty.

Together, these two events galvanized support for the war among the American populace and led to President Woodrow Wilson's decision to ask Congress for a declaration of war against Germany in April 1917. Thus, the combination of the Lusitania incident and the Zimmermann Telegram was pivotal in bringing the United States into the conflict.