The significant event that led to the U.S. entry into World War I was the sinking of the RMS Lusitania in May 1915, combined with the resumption of unrestricted submarine warfare by Germany in early 1917. The Lusitania, a British ocean liner with American passengers, was torpedoed by a German U-boat, resulting in the deaths of 128 Americans. This led to increased public outrage in the United States. Additionally, Germany’s subsequent actions, including the interception of the Zimmermann Telegram, which proposed a military alliance with Mexico against the U.S., prompted President Woodrow Wilson to ask Congress for a declaration of war in April 1917.