On an unknown date in the latter half of 782, Charlemagne did some seriously nasty business with a captured army of Saxons. In the late 8th century, the King of the Franks was fighting a decades-long running campaign against Saxon tribes at the edge of his sprawling European empire. One part Christianizing the pagans and two (or more) parts territorial aggrandizement, Charlemagne’s exertions on the Rhinish frontier were opposed by Widukind, or Witikind, or Widochind, whose “forest-child” name belies its owner’s legendary ferocity. In the summer of 782, when Charlemagne perhaps thought he had whatever passed for peace among the...