The Crusades occurred between the eleventh and fourteenth centuries. There were religious, political, and economic reasons for them. But religious reasons were paramount. Crusaders had different motivations, and some merely sought travel and adventure. Crusaders were often barbaric: they sacked cities and murdered Jews.

Alexius, the emperor of the Byzantine Empire, wanted to conquer Anatolia. His army was too weak, so he asked Pope Urban II for help. Alexius had merely wanted several thousand knights that he could use as mercenaries.

The rise of Muslim states in the East was increasingly viewed as a threat to the West, too. Muslims' control of Jerusalem made the West more receptive to Alexius's plea for help.

Pope Urban II had both political and religious reasons for helping Alexius. At the time of the Crusades, the Pope was both a political and religious leader. Urban saw an opportunity to strengthen papal secular authority in Europe. He also wanted to end the schism of 1054 with the church in the East. His call for a Crusade led to a massive movement that ultimately sent many thousands of West Europeans to the Holy Land.

Crusaders gained absolution for their sins and the promise of going to heaven as a reward if they died on a Crusade. The Crusade was the quintessential pilgrimage in a European society that was very religious. Only a few Crusaders, the great lords, had an opportunity to win lands in the East.

There were also economic reasons for the Crusades. Some Europeans, especially Italian city-states, sought greater trade with the East.

The religious motivations behind the Crusades were to recapture the Holy Land in the name of Christendom. This was important, as the Church considered this a fulfillment of prophecy which had to take place before Jesus could return. There was also a possibility of a reconciliation with the Byzantine Church, though this became more remote, as in the thirteenth century Constantinople would be razed by crusaders.

In medieval Europe, religion, economics, and politics were often interwoven, and it is a little hard to differentiate between the three. The Catholic Church wanted to obtain holy artifacts such as saints' bones and relics from Jesus's life. This could only be done by establishing kingdoms in the Levant. In many cases churchgoers would pay great sums in order to obtain and see these relics, because they were said to have healing powers. There was also the possibility of trade, especially if one could cut in on the lucrative spice and silk trade that was the exclusive domain of the Byzantine Empire and Venetian traders who were fine with trading with both Muslims and Christians. The Church also wished to reestablish itself as the leader of Christendom. Many upstart kings and nobles were fighting among themselves, and the Pope called forth a Crusade in order to demonstrate his power over the secular world. He also hoped that a war against Muslims in the Middle East would prevent Muslim incursions into Europe.

The religious motivation for the Crusades is the one most people know about: Europe was predominantly Christian and ruled by Christian governments while the Middle East was predominantly Muslim and ruled by Muslim governments. People in each religion believed (as many still do today) that their religion is the one true religion and everyone else's religion is wrong; furthermore, a substantial proportion believed that those who do not believe the right religion should suffer or even be put to death. Thus, there were a large number of Christians willing to kill Muslims simply because they were Muslims, and conversely a large number of Muslims willing to kill Christians simply because they were Christians.

But that is not the whole story of the Crusades.

There was also a substantial political motivation; European governments found that by launching wars and conquering territory in the Middle East they could strengthen their own power at home. In particular, the Catholic Church used the Crusades as a means of uniting all Christians in Europe under one banner and solidifying the authority of the Church in public life. There were strategic reasons to establish military footholds in the Middle East, particularly as a bulwark against the rise of Turkey.

Finally, there were economic motivations. The Middle East has always been a region very rich in natural resources (today we think in terms of oil, but back then petroleum was basically useless; they were more interested in precious metals like gold and silver, as well as simply rich farmland and comfortable living space). European governments reasoned that by capturing territory in the Middle East they could secure access to these natural resources and thereby make themselves wealthier. To some extent this was true, though they probably would have actually made more wealth by trading peacefully with Middle Eastern cultures rather than going to war to conquer them.