When World War I broke out, the United States initially maintained a policy of isolationism. They avoided conflict while trying to negotiate peace between the warring nations. When a German U-boat sank the British liner "Lusitania" in 1915 there were 128 Americans aboard. U.S. President Woodrow Wilson vowed that "America isn't too proud to fight". He 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. In 1917, three years after the first shots of the war were fired, the United States entered the war. By the end of the conflict more than 4.7 million American soldiers, sailors and Marines would fight in the war.