The significance of the sinking of the Lusitania in 1915 to the United States' entry into World War I lies in its impact on public opinion. The Lusitania, a British ocean liner, was torpedoed by a German U-boat, resulting in the deaths of 1,198 passengers, including 128 Americans. This event galvanized anti-German sentiment in the U.S. and highlighted the dangers of unrestricted submarine warfare. Although the U.S. remained neutral initially, the Lusitania sinking deepened calls for intervention, ultimately contributing to America's decision to join the Allies in 1917.