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

The event that triggered the U.S. entry into World War I was the Zimmermann Telegram. This was a secret communication sent by German Foreign Minister Arthur Zimmermann to Mexico in January 1917, proposing a military alliance between Germany and Mexico if the United States entered the war against Germany. The telegram was intercepted and decoded by British intelligence and made public, which caused widespread outrage in the United States. Unlike the sinking of the Lusitania, which also influenced public opinion due to loss of American lives but occurred two years earlier, the Zimmermann Telegram directly showed a threat to U.S. sovereignty and national security. The revelations from the telegram played a crucial role in galvanizing public support for U.S. involvement in the war, leading to President Woodrow Wilson asking Congress to declare war on Germany in April 1917.