King John (1166-1216) has long been seen as the epitome of bad kings. The son of the most charismatic couple of the middle ages, Henry II and Eleanor of Aquitaine, and younger brother of the heroic crusader king, Richard the Lionheart, John lived much of his life in the shadow of his family. When in 1199 he became ruler of his family's lands in England and France, John proved unequal to the task of keeping them together. Early in his reign he lost much of his continental possessions, and over the next decade would come perilously close to losing his English kingdom, too.

In King...