In your high school world history classes you may have been taught that the Crusades were an unprovoked assault on peaceful Muslims carried out by a greedy European church and aristocracy in order to plunder and pillage. You may also have been taught that the Crusaders were brutal barbarians who, though usually outclassed by a superior foe, often managed to massacre highly cultured, tolerant Muslims whose armies fought only to defend themselves, their lands, and their religion. Perhaps you also learned that it's the bitter memory of those Crusades that fuels modern Islamic resentments toward the West.
That's the narrative that has dominated Western education for a century and it's the narrative perpetuated by films like Kingdom of Heaven and by the media in the wake of 9/11, but as historian Rodney Stark argues in God's Battalions, none of it is true.
In his book, which often reads like a novel, Stark documents the provocations which led to the Crusades, how for four hundred years Muslims had attacked, killed, raped, and enslaved Christian pilgrims to the Holy Land and slaughtered Christian populations in cities that had been Christian from the earliest days of the church. Muslim armies swept across North Africa, conquering Italy, Sicily, and much of Spain, putting tens of thousands to the sword and enslaving women and children. Mohammed himself was responsible for the mass beheadings of 700 Jews in Medina who were first made to dig their own graves.
(This is not to say that the Crusaders were not also guilty of atrocities. German forces slaughtered Jews in the Rhineland at the outset of the first and second Crusades despite the frantic effort of Catholic bishops to save the victims.)
During the Crusades themselves Muslim troops, despite having superior numbers and the advantage of short supply lines, were rarely a match for the technologically superior Crusaders. The Europeans won almost every engagement against their Islamic foes, and were ultimately defeated, after over 200 years of protecting the Holy Lands, not because of Muslim skill and tactics but because people in Europe grew weary of sustaining the expense necessary to field an army so far from home.
The movie Kingdom of Heaven portrays the Europeans defending Jerusalem as largely thuggish savages and the Muslims under Saladin as compassionate and merciful. The facts are otherwise. Saladin was a ruthless killer responsible for numerous massacres both before and after his victory at Jerusalem, a victory due largely to the fact that the city was defended by only two knights and a multitude of untrained refugees and other "civilians".
It was the custom at the time that if a city surrendered without the besieging army having to take it by force, the defenders would be given safe passage. The defenders of Jerusalem requested such terms and Saladin granted them. He allowed about half of the inhabitants to pay a ransom and leave unmolested, but not because he was particularly chivalrous. Indeed, he enslaved about half of those who couldn't afford the price. Moreover, at the battle of Hattin he looked on with glee as his underlings took turns beheading captured Crusader knights, and even participated himself in the butchery.
If you enjoy reading history I highly recommend God's Battalions. Not only does it illuminate important events and dispel numerous myths about the Crusades, it's also a very enjoyable, even fascinating, read.