Map of the First Crusade
Religious War Launched by Christians Against Muslims
Launched by Pope Urban II at the Council of Clermont in 1095, the First Crusade was the most successful. Urban gave a dramatic speech urging Christians to swarm towards Jerusalem and make it safe for Christian pilgrims by taking it away from the Muslims. Jerusalem had no natural resources which Europeans needed; instead, it's "holiness" meant that Christians needed to control it rather than infidel Muslims. It should be little wonder, then, that secular critics of religion focus so much on the Crusades as a prime example of what's wrong with religion and why religion can be so dangerous.
Urban promised that those who agreed to take up the cross and participate in a religious war against Muslims would have their sins forgiven. This sanctified the violence which was to be caused, transforming it into a holy act designed to take control over a holy site. There was little economic value to the Crusades, aside perhaps from protecting trade routes, but the religious value to leaders of the Christian church was immeasurable. There is little difference between this doctrine of holy violence and the belief of many Muslims today that dying as a martyr will ensure one's entry into heaven.
The great success of the First Crusade served as an inspiration to all later religious wars launched by European Christians against Muslims in the Middle East. No other Crusade was nearly as successful, and most were utter failures, but participants in all were convinced that with enough religious faith they would be sure the repeat the same successes and conquer all the lands around Jerusalem. Faith, not military tactics, diplomacy, or politics was the driving force here.
Without pressure being applied by popes and other religious authorities on political leaders to protect, defend, or conquer the Holy Lands, most would have stayed home to concentrate on their own lands, relatives, and problems. Indeed, recalcitrant kings who didn't launch crusades fast enough were a problem for several popes. It didn't take long for secular leaders to realize that military expeditions against the Muslims cost far more than they could possibly gain, but popes and the church sacrificed little of their own and refused to give up he use of violence to pursue conquest over Islam.
The armies of the First Crusade left European cities in 1096 and finally captured Jerusalem in 1099. Devastation was caused all along the way - first among Jews in Europe then later in Muslim areas. From these conquered lands Crusaders carved out small kingdoms for themselves which endured for some time, though not long enough to have a strong, long-lasting impact upon local culture. This map depicts the basic route taken by the Crusading armies and the major cities which they captured on their way to Jerusalem. You can also see the small and vulnerable size of the Latin Kingdom of Jerusalem.