The Crusader States, By Malcolm Barber. Yale, £25

Between 1099 and 1192, an assortment of Western European Christians known as Franks or Crusaders managed to recapture, rule and eventually lose Jerusalem, Antioch, Edessa, Tripoli and the lands surrounding them in Syria, Palestine and the Levant. Known as Outremer, these were Latin and Catholic polities in lands then dominated by Greek Orthodox, or Shia and Sunni Muslim. Malcolm Barber's detailed, fair-minded and scholarly history of this collection of western states in an eastern setting sheds much light on both the period, and its repercussions, which are still with us today.

Then as now Shia and Sunni were at one another's throats, many assisting the Crusaders. Intense religious fervour and a belief that death in battle with the infidel granted a place in paradise enabled the Frankish armies to tolerate appalling casualties. Of the 100,000 who left on the First Crusade, fewer than 14,000 survived to recapture Jerusalem. For the Franks, the lands they sought to reconquer had been Christian until the Turkish and Egyptian Muslim invasions. Jerusalem was the centre of the medieval world and of Christendom.

Outremer was to be a springboard for the Christian conquest of Egypt, Mesopotamia and Persia. The great nobles were already jockeying for who would rule these rich lands. It was not to be. Christian Outremer was to be a temporary settlement. A cultural history as well as an account of seige and battle, Barber's book explains why this was. There were never enough Crusaders; they never had enough money; their supply lines were precarious and too long.

Reinforcements and money had to be moved 2,700 miles overland, or equally riskily by sea. The overland route passed through Byzantine Greek territory and the emperors in Constantinople were lukewarm at best, hostile at worst. Illness and disease weakened their forces. And in Saladin, the Muslims found a commander of genius. Given the poor odds, it is surprising Outremer lasted as long as it did.

Robert Carver's 'Paradise With Serpents: Travels in the lost world of Paraguay' was shortlisted for the 2008 Ondaatje Award