A submarine sank the RMS Lusitania, is the right answer.
The most significant reason why the United States entered the First World War was the sinking American ships by Germany. Following the seizure of the U.S. Vessels, the then U.S. President Woodrow Wilson announced Germany's violation of its pledge to halt authorized submarine warfare in the North Atlantic and the Mediterranean, including its efforts to allure Mexico into commitment against the U.S. as his plans for opposing the war. In short, the seizure of American ships led the United States to enter the First World War.
