Mention the Vikings and most people will immediately think of horned helmets, blood eagles, and grizzled barbarians raiding and pillaging their way across the seven seas. It's a cliché, of course, but one embedded with an element of truth. During the early years of the Viking Age (circa 800–1100) the Vikings were engaged in a campaign of what we might now term "asymmetrical warfare," attacking religious targets in a savage attempt to assert their own culture against Charlemagne's expanding Christian empire.

Yet these...