Why were the Crusades launched? Were the Crusades primarily religious, political, economic, or a combination? There is a wide variety of opinion on this matter. Some argue that they were a necessary response by Christendom to the oppression of pilgrims in Muslim-controlled Jerusalem. Others claim that it was political imperialism masked by religious piety. Still others argue that it was a social release for a society that was becoming overburdened by landless nobles.
Christians commonly try to defend the Crusades as political or at least as politics being masked by religion, but in reality sincere religious devotion — both Muslim and Christian — played a primary role on both sides. It's little wonder that the Crusades are so often cited as a reason to regard religion as a cause for violence in human history. The most immediate cause for the Crusades is also the most obvious: Muslim incursions into previously Christian lands. On multiple fronts, Muslims were invading Christian lands to convert the inhabitants and assume control in the name of Islam.
A "Crusade" had been underway on the Iberian peninsula since 711 when Muslim invaders conquered most of the region. Better known as the Reconquista, it lasted until the tiny kingdom of Grenada was reconquered in 1492. In the East, Muslim attacks on land controlled by the Byzantine Empire had been going on for a long time. After the battle of Manzikert in 1071, much of Asia Minor fell to the Seljuk Turks, and it was unlikely that this last outpost of the Roman Empire would be able to survive further concentrated assaults. It wasn't long before the Byzantine Christians asked for help from Christians in Europe, and it's no surprise that their plea was answered.
A military expedition against the Turks held out a lot of promise, not least of which was the possible reunification of the Eastern and Western churches, should the West prove capable of defeating the Muslim menace which had for so long plagued the East. Thus the Christian interest in the Crusades was not only to end the Muslim threat, but also to end the Christian schism. Aside from that, however, was the fact that if Constantinople fell then all of Europe would be open to invasion, a prospect that weighed heavily on the minds of European Christians.
Another cause for the Crusades was the increase in problems experienced by Christian pilgrims in the region. Pilgrimages were very important to European Christians for religious, social, and political reasons. Anyone who successfully made the long and arduous journey to Jerusalem not only demonstrated their religious devotion, but also became beneficiaries of significant religious benefits. A pilgrimage wiped clean one's plate of sins (sometimes it was a requirement, the sins were so egregious) and in some cases served to minimize future sins as well. Without these religious pilgrimages, Christians would have had a harder time justifying claims to ownership and authority over the region.
The religious enthusiasm of the people who went off on the Crusades can't be ignored. Although there were a number of distinct campaigns launched, a general "crusading spirit" swept across much of Europe for a long time. Some Crusaders claimed to experience visions of God ordering them to the Holy Land. These usually ended in failure because the visionary was typically a person without any political or military experience. Joining a Crusade was not simply a matter of participating in military conquest: it was a form of religious devotion, particularly among those seeking forgiveness for their sins. Humble pilgrimages had been replaced by armed pilgrimages as church authorities used the Crusades as part of the penance people had to do to repay sins.
Not all of the causes were quite so religious, though. We know that the Italian merchant states, already powerful and influential, wished to expand their trade in the Mediterranean. This was being blocked by Muslim control of many strategic seaports, so if Muslim domination of the eastern Mediterranean could be ended or at least significantly weakened, then cities like Venice, Genoa, and Pisa had a chance to enrich themselves further. Of course, richer Italian states also meant a richer Vatican.
In the end, the violence, death, destruction, and continuing bad blood that last through to the present day would not have occurred without religion. It doesn't matter so much who "started it," Christians or Muslims. What matters is that Christians and Muslims eagerly participated in mass murder and destruction, mostly for the sake of religious beliefs, religious conquest, and religious supremacism. The Crusades exemplify the way in which religious devotion can become a violent act in a grand, cosmic drama of good vs. evil — an attitude which persists through today in the form of religious extremists and terrorists.