Was the First Crusade caused by religion or self-interest?

The short answer

• Manzikert and the Byzantine appeal. The defeat at Manzikert in 1071 was followed by the loss of much of Anatolia, a region that had supplied Byzantium with food and soldiers. Alexios I Komnenos sought Western mercenaries to recover it. Urban’s appeal brought tens of thousands east, pursuing a holy war with aims of their own. The emperor had asked for troops to employ; the movement he received was far harder to direct.
• The Seljuk fracture. After Malik-Shah died in 1092, his heirs and their atabegs fought over his realm. Rival emirs held Syria, while the Sunni-Shia division set Baghdad apart from Fatimid Cairo. These rulers had reasons to distrust one another when the expedition arrived. A united defense might have stopped it at the frontier; their divisions made such a response harder to organize.
• The reform papacy and the Peace of God. Gregorian reform had strengthened a papacy able to make demands on kings. The Peace and Truce of God movements, meanwhile, tried to limit knights’ private wars. Urban joined papal authority to a new purpose for those warriors: fighting eastward could become an act of penance.
• Jerusalem and the pull of pilgrimage. Pilgrims already traveled in growing numbers to Jerusalem, the center of the medieval Christian map. Urban’s armed journey drew on that practice as well as apocalyptic expectations and the promise of remission of sins for those who died on the road. These hopes gave the expedition a religious force beyond an ordinary call to political service.

What it changed

An armed pilgrimage, not an army. The people who set out were bound by vows rather than organized as a single state army. Lords raised forces and often sold or mortgaged land to fund the journey; poorer people went with them. Their commitment helped sustain the movement, while the absence of a common commander and budget made supply and coordination difficult.

A new spiritual technology. Crusading helped develop the indulgence, the remission of the temporal penalty for sin. Later appeals would turn the idea against heretics, pagans and the pope’s political enemies (Ch. 9). The sale of indulgences later became a dispute associated with the Reformation.

A blow the target barely felt. The first Islamic responses to the invaders were slow and largely local. Over roughly fifty years, leaders including Zengi and Nur al-Din built a broader campaign against the Latin states. Before such coalitions formed, the Latins had time to establish small possessions.