When World War I was raging in Europe, German U-Boats (submarines) infested the waters off the British Isles. The Lusitania, a luxury cruise ship, was sailing from New York to England in 1915. Some passengers worried; most did not. Cunard, the venerable British cruise line that owned the ship, was confident that all would remain safe. After all, the Lusitania was big, strong, and fast. One of the speediest passenger vessels of its era, the Lusitania could slice through the ocean at 25 knots....