U.S. pressure to enter World War I mounted in May of 1915 after a German submarine sank the Lusitania, a British passenger liner, which killed over 1,000 people, 128 of whom were Americans. The German government subsequently resorted to unrestricted submarine warfare, which led the U.S. to enter the war in April of 1917. America's entry into the war helped boost the Allied troops' morale and contributed much-needed industrial resources and personnel to the war effort. It also provided American dime novels and British penny dreadfuls with a host of new patriotic subject matter.