On May 1, 1915, a luxury ocean liner as richly appointed as an English country house sailed out of New York, bound for Liverpool, carrying a record number of children and infants. The passengers were anxious. Germany had declared the seas around Britain to be a war zone, and for months, its U-boats had brought terror to the North Atlantic. But the Lusitania was one of the era's great transatlantic "Greyhounds" and her captain, William Thomas Turner,...