When World War I broke out, the United States maintained a policy of isolationism, avoiding conflict while trying to negotiate peace between the warring nations. However, when a German U-boat sank the British liner "Lusitania" in 1915, with 128 Americans aboard, U.S. President Woodrow Wilson demanded an end to attacks on passenger ships. Germany complied and Wilson unsuccessfully tried to mediate a settlement. He repeatedly warned that the U.S. would not tolerate unrestricted submarine warfare, in violation of international law.