NO sooner had the knights of the First Crusade captured Jerusalem from the Seljuk Turks in 1099 than writers began to swoon over their achievements. Inspired by a rousing call by Pope Urban II at Clermont, France, four years earlier to rescue the Holy Land, these first historians wrote, the crusaders and their conquest of the eastern Mediterranean coast proved that God had smiled on western Europe and the worldly authority of Rome.

That story, and the papal authority it underlined, shaped the next 500 years of European history. Even today, the idea at the center of the crusades, that religion has long been at the heart of the East-West divide, drives foreign policy from Washington to Islamabad. But the real story is much more complicated, and much more earthly, than most people recognize.

The subject of the crusades, and in particular the first, has received enormous attention from scholars over the centuries, to the point that one leading historian wrote in a recent book review that there was nothing original left to say: the story is too well known, too secure.

Yet for all that work, distortions remain. The armchair historian could be forgiven for thinking, for example, that Jerusalem fell to the Muslims soon before the First Crusade set out to supposedly rescue it. In fact, Jerusalem fell some 450 years earlier.