The First Crusade (1096–1099) was the first of a series of religious wars, or Crusades, which were initiated, supported and at times directed by the Latin Church in the Middle Ages. Their aim was to return the Holy Land—which had been conquered by the Rashidun Caliphate in the 7th century—to Christian rule. By the 11th century, although Jerusalem had then been ruled by Muslims for hundreds of years, the practices of the Seljuk rulers in the region began to threaten local Christian populations, pilgrimages from the West and the Byzantine Empire itself. The earliest impetus for the First Crusade came in 1095 when Byzantine emperor Alexios I Komnenos sent ambassadors to the Council of Piacenza to request military support in the empire's conflict with the Seljuk-led Turks. This was followed later in the year by the Council of Clermont, at which Pope Urban II gave a speech supporting the Byzantine request and urging faithful Christians to undertake an armed pilgrimage to Jerusalem.
This call was met with an enthusiastic popular response across all social classes in western Europe. Thousands of predominantly poor Christians, led by the French priest Peter the Hermit, were the first to respond. What has become known as the People's Crusade passed through Germany and indulged in wide-ranging anti-Jewish activities, including the Rhineland massacres. On leaving Byzantine-controlled territory in Anatolia, they were annihilated in a Turkish ambush led by the Seljuk Kilij Arslan I at the Battle of Civetot in October 1096.
In what has become known as the Princes' Crusade, members of the high nobility and their followers embarked in late-summer 1096 and arrived at Constantinople between November and April the following year. This was a large feudal host led by notable Western European princes: southern French forces under Raymond IV of Toulouse and Adhemar of Le Puy; men from Upper and Lower Lorraine led by Godfrey of Bouillon and his brother Baldwin of Boulogne; Italo-Norman forces led by Bohemond of Taranto and his nephew Tancred; as well as various contingents consisting of northern French and Flemish forces under Robert Curthose of Normandy, Stephen of Blois, Hugh of Vermandois, and Robert II of Flanders. In total and including non-combatants, the forces are estimated to have numbered as many as 100,000.
The crusader forces gradually arrived in Anatolia. With Kilij Arslan absent, a Frankish attack and Byzantine naval assault during the Siege of Nicaea in June 1097 resulted in an initial crusader victory. In July, the crusaders won the Battle of Dorylaeum, fighting Turkish lightly armoured mounted archers. After a difficult march through Anatolia, the crusaders began the Siege of Antioch, capturing the city in June 1098. Jerusalem, then ruled by the Fatimids, was reached in June 1099, and the ensuing Siege of Jerusalem culminated in the Crusader armies storming and capturing the city on 15 July 1099, during which assault a large fraction of the residents were massacred. A Fatimid counterattack was repulsed later that year at the Battle of Ascalon, which marked the end of the First Crusade. Afterwards, most of the crusaders returned home.
Four Crusader states were established in the Holy Land: the Kingdom of Jerusalem, the County of Edessa, the Principality of Antioch, and the County of Tripoli. The Crusaders maintained some form of presence in the region until the loss of the last major Crusader stronghold in the 1291 Siege of Acre, after which there were no further substantive Christian campaigns in the Levant.
Christian and Muslim states had been in conflict since the establishment of Islam in the 7th century. In the span of approximately 120 years after the death of the Islamic prophet Muhammad in 632, Muslim forces conquered the Levant (including Jerusalem), as well as North Africa and most of the Iberian Peninsula, all of which had previously been under Christian rule. By the 11th century, Christians were—through the Reconquista—gradually reversing the 8th-century Muslim conquest of Iberia, but their ties to the Holy Land had deteriorated. Muslim authorities in the Levant often enforced harsh rules against any overt expressions of the Christian faith. Approximately two-thirds of land held by Christians had been conquered by Muslim forces prior to the First Crusade.
The First Crusade was the response of the Christian world to the expansion of Islam, due to the Fatimids and Seljuks, into the Holy Land and Byzantium. In Western Europe, Jerusalem was an increasingly important destination for Christian pilgrimages. While the Seljuk hold on Jerusalem was weak (the group later lost the city to the Fatimids), returning pilgrims reported difficulties and the oppression of Christians. The Byzantine need for military support coincided with an increase in the willingness of the western European warrior class to accept papal military command.
By the 11th century, the population of Europe had increased greatly as technological and agricultural innovations allowed trade to flourish. The Catholic Church had become a dominant influence on Western civilisation. Society was organised by manorialism and feudalism, political structures whereby knights and other nobles owed military service to their overlords in return for the right to rent from lands and manors.
In the period from 1050 until 1080, the Gregorian Reform movement developed increasingly more assertive policies, eager to increase its power and influence. This prompted conflict with eastern Christians rooted in the doctrine of papal supremacy. The Eastern church viewed the pope as only one of the five patriarchs of the Church, alongside the patriarchates of Alexandria, Antioch, Constantinople and Jerusalem. In 1054, theological disputes over Papal Supremacy, Eucharistic practices, and the insertion of the Filioque Clause into the Nicene Creed prompted Pope Leo IX to send a legation to Patriarch Michael I Cerularius of Constantinople, which ended in mutual excommunication and the East–West Schism.
Early Christians were used to the employment of violence for communal purposes. A Christian theology of war inevitably evolved from the point when Roman citizenship and Christianity became linked. Citizens were required to fight against the empire's enemies. Dating from the works of the 4th-century theologian Augustine of Hippo, a doctrine of holy war developed. Augustine wrote that aggressive war was sinful, but war could be justified if proclaimed by a legitimate authority such as a king or bishop, it was defensive or for the recovery of lands, and it did not involve excessive violence. The breakdown of the Carolingian Empire in Western Europe created a warrior caste who now had little to do but fight amongst themselves. Violent acts were commonly used for dispute resolution, and the papacy attempted to mitigate it.
Pope Alexander II developed recruitment systems via oaths for military resourcing that Pope Gregory VII further extended across Europe. These were deployed by the Church in the Christian conflicts with Muslims in the Iberian Peninsula and for the Norman conquest of Sicily. Gregory went further in 1074, planning a display of military power to reinforce the principle of papal sovereignty in a holy war supporting Byzantium against the Seljuks, but was unable to build support for this. Theologian Anselm of Lucca took the decisive step towards an authentic crusader ideology, stating that fighting for legitimate purposes could result in the remission of sins.
On the Iberian Peninsula, there was no significant Christian polity. The Christian realms of León, Navarre, and Catalonia lacked a common identity or shared history based on tribe or ethnicity, resulting in frequent periods of unity and division during the 11th and 12th centuries. Although small, all developed an aristocratic military technique and, in 1031, the disintegration of the Caliphate of Córdoba in southern Spain created the opportunity for the territorial gains that later became known as the Reconquista. In 1063, William VIII of Aquitaine led a combined force of French, Aragonese and Catalan knights in the Siege of Barbastro, taking the city that had been in Muslim hands since the year 711. This had the full support of Alexander II, and a truce was declared in Catalonia with indulgences granted to the participants. It was a holy war but differed from the First Crusade in that there was no pilgrimage, no vow, and no formal authorisation by the church. Shortly before the First Crusade, Urban II had encouraged the Iberian Christians to take Tarragona, using much of the same symbolism and rhetoric that was later used to preach the crusade to the people of Europe.