In November 1095, in a field outside the French town of Clermont, a pope called on Western knights to march east. The response surprised even him. Pope Urban II launched the First Crusade at the Council of Clermont in France in 1095, answering an appeal for help from the Byzantine emperor Alexios I, whose lands in Asia Minor had been overrun by the Seljuk Turks after the Byzantine defeat at Manzikert in 1071. Urban turned a request for soldiers into a holy war to free Jerusalem, promising that the journey would count as full penance for the sins of those who went.

The First Crusade

Before the organized armies set out, popular bands gathered behind preachers such as Peter the Hermit. Some turned first on Jews at home. In 1096 crusading bands massacred Jewish communities in Rhineland cities such as Speyer, Worms and Mainz, sometimes despite local bishops' efforts to protect them. Hebrew chronicles record families who chose death over forced baptism.

The main armies crossed into Asia Minor in 1097, took Antioch after a brutal siege and stormed Jerusalem in July 1099, killing much of its Muslim and Jewish population. The victors set up four crusader states: the County of Edessa, the Principality of Antioch, the County of Tripoli and the Kingdom of Jerusalem. Military orders of warrior monks, the Templars and Hospitallers, formed to defend them. For almost a century Franks, local Christians, Muslims and Jews lived side by side under crusader rule, trading and sometimes adopting each other's customs, while frontier warfare went on around them.

The Muslim recovery

The crusaders succeeded partly because the Muslim rulers of the region were divided. Unity came slowly. Zengi took Edessa in 1144, and his son Nur al-Din promoted holy war against the Franks. Then the Kurdish sultan Saladin, ruling Egypt and Syria, destroyed the crusader army at Hattin and retook Jerusalem in 1187. Unlike the crusaders in 1099, he let most inhabitants ransom themselves and leave. The Third Crusade, led by Richard I of England and Philip II of France, regained coastal cities but not Jerusalem; a truce in 1192 let Christian pilgrims visit.

Later crusades went badly. The Fourth was diverted to sack Constantinople in 1204, a disaster for the Byzantines. Crusading ideas were turned against pagans in the Baltic, Muslims in Spain and heretics in France. The fall of Acre in 1291 ended the crusader states in the Holy Land.

Why did they go?

Older histories often described crusaders as landless younger sons seeking their fortunes. Jonathan Riley-Smith argued that most crusaders were moved by piety, pointing out that crusading was ruinously expensive and many sold or mortgaged land to go. Charters show families pooling resources for a journey from which many never returned. Other historians stress that piety, honor, family duty and hope of gain were mixed together, and that religious violence was itself part of the faith of the time.

Other perspectives

The Byzantines, who had asked for help, soon found their helpers alarming. Anna Komnene, the emperor Alexios's daughter, portrayed the crusaders in her history as brave but greedy, crude and untrustworthy. Her book, the Alexiad, is one of the few histories written by a medieval woman.

For Muslims in Syria, the Franks were at first one more enemy among many, and daily contact could be surprisingly ordinary. Usama ibn Munqidh, a Syrian nobleman, left a memoir describing both fighting and friendships with Franks, along with scornful stories about Frankish medicine and justice. Historians such as Carole Hillenbrand have studied how Muslims remembered these wars. The common Arabic term for "the Crusades" came into wide use only in the 1800s. Before then Muslim writers mostly spoke simply of wars with the Franks, and the Mongol invasions loomed much larger in memory. Modern memory of the crusades, in both the West and the Middle East, owes a great deal to the age of European empires.

Worked example: two accounts of Jerusalem, 1099

Raymond of Aguilers, a priest who marched with the crusade, wrote that in the Temple area the crusaders rode in blood up to their knees and bridle reins, and he called it a just judgment of God. More than a century later, the Muslim historian Ibn al-Athir wrote that the Franks killed more than 70,000 people in the al-Aqsa Mosque alone.

Both accounts are shaped by purpose. Raymond's image of blood to the bridle echoes the Book of Revelation and celebrates the slaughter as holy. Ibn al-Athir wrote when remembering the massacre served calls for unity against the Franks. Modern historians judge both the imagery and the figure exaggerated: the whole city may not have held that many people, and letters preserved in the Cairo Geniza show that some Jewish captives were ransomed. Yet they agree a large massacre took place. Reading opponents' accounts side by side, and asking what each writer wanted, is how historians reach a defensible account.