The Vikings gave no quarter when they stormed the city of Nantes, in what is now western France, in June 843—not even to the monks barricaded in the city's cathedral. "The heathens mowed down the entire multitude of priest, clerics, and laity," according to one witness account. Among the slain, allegedly killed while celebrating the Mass, was a bishop who later was granted sainthood.

To modern readers the attack seems monstrous, even by the standards of medieval warfare. But the witness account contains more than a touch of hyperbole, writes Anders Winroth, a Yale history professor and author of the book The Age of the Vikings, a sweeping new survey. What's more, he says, such exaggeration was often a