Causes were layered, not single
No single factor explains why thousands of people left home for Jerusalem in 1096. Historians usually describe a combination of religious belief, political events, social conditions and, to a lesser degree, economic pressures. The weight given to each is still discussed.
Religious causes
Western Christianity in the eleventh century placed great value on pilgrimage and penance. Jerusalem, as the place of Christ’s death and resurrection, was the most sacred destination of all. Church reformers also sought to channel the energy of the warrior class, which they considered violent, toward acceptable ends. Earlier movements, such as the Peace of God, had tried to limit violence against clergy and the poor.
Pope Urban II combined these themes. He presented the expedition as a penitential journey that would earn forgiveness for sins, and as a defense of fellow Christians in the East. This promise of spiritual reward proved powerful, and was a new element in the history of warfare.
Political causes
The Byzantine emperor Alexios I had asked for Western help after losing much of Anatolia to the Seljuk Turks. The Seljuk empire itself was fragmenting after the 1090s, which left Syria divided among rival rulers. That division helped the crusaders, but it was not the cause of their departure.
In the West, the papacy was competing with the German emperors in what is known as the Investiture Controversy. Leading a successful expedition raised the pope’s prestige as the head of Christendom, and historians often see this as a background motive, though not necessarily a conscious one for Urban himself.
Social and family causes
Medieval noble families had strong ties of kinship and lordship, and many crusaders traveled with relatives and followers. Younger sons who did not inherit land may have seen opportunity in the East, although many crusaders were in fact established lords who spent large sums and sometimes mortgaged land to go. The idea that crusading was mainly an outlet for landless knights is now treated with caution.
Some ordinary people were moved by preaching, hardship or visions of an approaching end of the world. Bad harvests and local famines in the 1090s may have increased unrest, though the evidence for this is debated.
Economic causes
Italian ports such as Genoa, Pisa and Venice supplied ships and later won trading privileges in the East. These cities profited, but their first role was transport. Economic motives were probably less important in 1095 than for later crusades, when Venice’s commercial interests clearly influenced decisions.
Consequences in the Near East
For the Latin settlers, the immediate result was the creation of the Crusader States. For the Muslim world, the crusades were at first treated as a regional threat rather than a major religious event. Over time, preaching about jihad grew stronger, particularly under Zengi, Nur al-Din and Saladin, who built their authority partly on opposition to the Franks.
Local Eastern Christians, Jews and Muslims all experienced the conflict differently. The capture of Jerusalem in 1099 was followed by killings of many inhabitants, and Jewish communities in the Rhineland had already been attacked in 1096. These events left long memories.
The sack of Constantinople in 1204 deepened the split between Western and Eastern Christians and left the Byzantine Empire weaker.
Consequences in Europe
Europe felt the effects more slowly. Papal authority grew as popes organized and financed crusades, and new forms of taxation were introduced to pay for them. The military orders, founded to protect pilgrims and care for the sick, became wealthy institutions. Italian trading cities expanded their networks. Contact with Eastern products and ideas increased, although much learning from the Islamic world arrived through Spain and Sicily rather than the crusades.
The crusading idea was also turned inward, against heretics and political enemies of the papacy, which helped normalize the use of religious war in Europe.
Judging the outcome
By the end of the thirteenth century, the territories had been lost and the central aim had failed. Yet crusading remained a model of religious violence and devotion for centuries. Today, historians stress that causes and consequences varied by period, and that generalizations about the whole movement should be made cautiously.