The popular understanding of crusades history is that ignorant, intolerant, and over-agressive Western Christians suddenly invaded the cultured, tolerant, and peaceful Muslim Near-East bent on killing everyone who wasn't a Christian. In this book, Rodney Stark sets out to refute every one of those assumptions. He argues that the crusades were not unprovoked, the Christian West was not a backwards culture, the Muslims were not more intellectually advanced or more tolerant, and the crusaders were no more brutal in their warfare than the Muslims that they were fighting. On most of these counts Stark makes a convincing case.
Stark begins his account where few crusades historians do: at the rise of Islam. I've read a great many books about the crusades (it's my field of study), and almost every single one begins either with the loss of Byzantine territory to Turkish Muslims in the second half 11th century or, even less helpfully, with the campaign of Pope Urban II just prior to the First Crusade. Stark notices this, and points out the very important fact that the Muslims attacked first, capturing the Holy Land, Egypt, North Africa, Sicily, and most of Spain from Christian control during the centuries prior to the First Crusade. The Crusaders were not simply trying to take territory from the Muslims, they were trying to take back territory the Muslims had taken from them by force during the great expansion of Islam in the early Middle Ages. It wasn't as if Christians suddenly decided to attack some innocent bystanders over in the Near East; the Near East had for centuries been Christian before bands of Islamic warriors took it by force from the Byzantine Empire. The West was further provoked by the recent (11th century) attacks on Christian pilgrims journeying to Jerusalem. They had been relative safe during the period when the Holy Land was controlled by Egypt, but when Turkish Muslims conquered it in the latter half of the 11th century they began killing Christians and disrupting the pilgrim route.
Stark then attacks the idea that Islamic culture in the Near East was significantly more advanced that Western Christian culture. He systematically goes through the major thinkers in Islamic culture who advanced learning, and nearly every one of them turns out to be either a Christian, Zoroastrian, Hindu, or Jew who happened to be living in territory ruled by Muslims. Even the most cherished Arabic discovery put forth to show the superiority of Islamic culture, Arabic numerals, was not Arabic at all, but Hindu, and was not even adopted among the Muslims except by its advanced mathematicians. The supposedly advanced Muslim medicine was actually the medicine of Nestorian Christians. The great feats of copying Greek philosophy and medicine were accomplished mostly by dhimmi, those living in the Islamic society who were not themselves Muslims. Almost everywhere in the Islamic empire, besides the Arabian Peninsula and Egypt, the vast majority of the population was not Muslim, but was simply ruled by a very small class made up of Muslims. Most of the academic accomplishments were done by the cultures over which a few Muslims happened to rule, not by Muslims themselves. And in the field of technology, Islamic culture lagged far behind the Christian West. Islamic fleets could not begin to compete with either Western fleets or Byzantine fleets. The reason the crusader army of only 20-40000 soldiers was able to come in and conquer the Holy Land from vastly larger Muslim forces was its significant technological superiority. The crossbow, plate armor, effective saddles, etc. allowed small groups of Western knights to demolish ill-equipped Muslim hordes. There is a reason the Muslims thought Christian knights were invincible before the disastrous Second Crusade showed that they could be stopped.
Also disproven is the popular notion that crusaders went East to gain wealth. It is (or at least was) common to hear that many who joined the crusades were families' younger sons, who had no prospects in the West. Instead, they turned their attention to the East in hopes of gaining loot and perhaps prestigious positions. Stark shows that it cost a great deal more than one could hope to gain just to equip oneself and make it to the Holy Land. It cost four to five times his annual income for a knight to go on a crusade; many people sold all that they had (including land) to fund their journey and bankrupted themselves before they even set out. The idea that people went East to gain money is absurd when one considers the cost of crusading.
Finally, Stark discusses the brutality of the crusaders. The most common example given to show that the crusaders were bloodthirsty brutes is the massacre following the capture of Jerusalem. The crusaders had besieged Jerusalem, which refused to surrender. When the crusaders finally breeched the walls months later, after suffering heavy casualties due to starvation and attacks from the city walls, they slaughtered everyone inside. Stark points out, quite correctly, that this was, in fact, standard procedure in medieval warfare, both for Christians and Muslims. An invading army suffered serious casualties when it was forced to keep a lengthy siege and storm a city, and so the common practice was to massacre cities that held out against a siege as an example to keep other cities from doing the same. If a city surrendered, however, the population was spared (of course, there were some out of control soldiers at times on both sides which sometimes broke this rule). It is ridiculous to compare the massacre at Jerusalem when the crusaders conquered it with the release of the citizens when Saladin reconquered for the Muslims a hundred years later; when the crusaders captured it Jerusalem had held out and cost the crusaders a great number of lives, and when Saladin captured it the Christians surrendered. Muslims were just as prone to massacre Christians, with the exception of Saladin, who was, from time to time, generous toward the crusaders (though he brutally massacred them too, from time to time). If one wishes to judge the crusaders by the modern conventions of warfare, then at least be consistent and judge the Muslims to have just as bad.
Most of the points Stark makes are very convincing, but much of it is not really new. You can find a refutation of the crusading-for-money theory in Jonathan Riley-Smith's The First Crusade and the Idea of Crusading. An account of why the crusades were, at least in part, a reaction to Muslim conquests and persecution of Christian pilgrims can be found in the writings of Thomas Madden or, again, Riley-Smith. Even the defense of the massacre at Jerusalem can be found in the work of crusades historian John France. Stark, however, is the first author I have seen to realize the significance of all the separate points and put them all together in one volume, and some of what he said I had not encountered elsewhere. In all the reading I have done on the crusades I had never once seen a historian note the significance of the crossbow (and other technological superiorities besides armor) to the success of the crusades (even R. C. Smail's standard work, Crusading Warfare, failed to discuss crossbows), and the fact that the Muslims ruled over a largely non-Muslim population had escaped my notice. Stark puts together a very good case, but he tends to go too far in many places. Instead of being content with demonstrating that the crusaders were not culturally or morally inferior to the Muslims, he tries to justify everything the crusaders did and present them as wholesome warriors of God. While the crusaders have been unduly vilified, largely as a result of anti-Catholic propaganda during the Enlightenment, they were certainly not all holy warriors seeking first the kingdom of God. Many of theme were ruthless, greedy, opportunistic exploiters. It is just as wrong to claim that they were all good as it is to claim that they were all evil. Stark has done much in this book to help us reach a more balanced appraisal of the crusades, but he has not quite achieved it himself.