When Count Henry of Anjou and his formidable wife Eleanor of Aquitaine became king and queen of England, they amassed an empire stretching 1,000 miles from the Pyrenees to the Scottish border, including half of France. Henry's grandmother Empress Mathilda of Germany had taught him that ruling is like falconry: show the hawk the reward, but take it away at the last moment, to keep the bird eager to please. To sons and vassals alike, Henry promised...