In 1861, the archaeologist James Farrer excavated a grass-grown mound on the Orkney Islands off the north coast of Scotland. He discovered a Neolithic tomb that had once provided shelter for the prehistoric dead. He also found evidence that many centuries earlier, Norse-speaking tomb-breakers had entered the mound and left dozens of inscriptions, one of which stood out: "Crusaders broke into this barrow."

The world of the Viking north and the era of the Crusades sit apart from one another in the imagination, divided by the mental gulf created by the Norman Conquest. The Vikings are seen as a phenomenon of the Dark Ages; the Crusades on the other hand are squarely medieval. Both images are false, of course, created and compounded by centuries of historiographical habit. From the tenth century onward, the men of the north were as likely to be found killing in the name of Christ as they were to be pillaging the monasteries of Britain, Ireland and France. And, by the eleventh and twelfth centuries, the swords of Scandinavia were turned with frequency and fervor against the perceived enemies of God: in the Holy Land, in the wild and frozen world of the eastern Baltic and within the bounds of their own Scandinavian realms.

God's Hammer is full of the adventure, horror, and madness associated with the Crusades, yet it exists in a world that remained inspired by the Viking heroes of the past. The focus throughout is on the deeds and experiences of the Scandinavians themselves; of the change they wrought upon the people and places they encountered, and the way in which the experience of Holy War transformed the self-image and mentality of those who prosecuted it and shaped the societies that gave rise to and sustained it.