Crusading movement
The Crusading movement—a major religious, political, and military endeavour of the Middle Ages—is conventionally dated from the Council of Clermont, at which Pope Urban II proclaimed an armed expedition in support of Eastern Christians under Muslim rule. He framed it as a form of penitential pilgrimage. By then, papal authority had grown through church reforms, and tensions with secular rulers encouraged the notion of holy war—combining classical just war theory, biblical precedents, and Augustine's teachings on legitimate violence. Armed pilgrimage aligned with the era's Christocentric and militant Catholicism, sparking widespread enthusiasm. Western expansion was further enabled by economic growth, the decline of older Mediterranean powers, and Muslim disunity. These factors allowed crusaders to seize territory and found four Crusader states in the Levant. Their defence inspired successive Crusades, and the papacy also launched crusading campaigns against other targets—Muslims in Iberia, pagans in the Baltic, and other opponents of papal authority.
Though aimed primarily at the warrior elite through appeals to chivalric ideals, the movement depended on broad support from clergy, townspeople, and peasants. Women, despite being discouraged, were involved as participants, proxies for absent crusaders, or victims. Although many crusaders were motivated by indulgences, material gain also played a part. Crusading campaigns were typically initiated through papal bulls, with participants pledging to join by "taking the cross"—sewing a cross onto their garments. Failure to fulfil vows could result in excommunication. Periodic waves of zeal produced unsanctioned "popular crusades".
The papal-sanctioned wars fostered distinctive institutions and ideologies. Initially funded through improvised means, later campaigns received more organized support via papal taxes on clergy and the sale of indulgences. Core crusading forces were heavily armed knights, backed by infantry, local troops, and naval aid from maritime cities. Crusaders secured their holdings by building powerful castles, and the fusion of chivalric and monastic ideals led to the rise of military orders. The movement extended Western Christendom and created new frontier states, some surviving into the early modern period. Crusading encouraged cultural exchange and left lasting marks on European art and literature. Despite a decline during the Reformation, anti-Ottoman "holy leagues" sustained the tradition into the 18thcentury.
Background
The Crusades are commonly defined as Christian religious wars waged by Western European warriors during the Middle Ages to capture Jerusalem. Related campaigns differed markedly in spatial reach, temporal limits, and motivating aims. The wider crusading movement fostered distinctive institutions and ideologies that shaped society in Catholic Europe and neighbouring regions.Classical just war theories
In classical antiquity, Greek philosophers and Roman jurists formulated just war theories that later influenced crusading theology. Aristotle stressed the need for a just end, asserting "war must be for the sake of peace". Roman law required a —just cause—and held that only legitimate authorities could declare war, with defence, restitution, and punishment considered acceptable grounds.Although the Bible—Christianity's core scripture—presents conflicting views on violence, the 4th-century Christianisation of the Roman Empire gave rise to Christian just war theory. Bishop Ambrose, a former imperial official, was the first to equate enemies of the Christian state with those of the Church.
The empire was divided in 395. Fifteen years later, the sack of the city of Rome led Augustine—Ambrose's student—to write The City of God in which he argued that the Bible's prohibition on killing did not apply to wars waged with divine approval. He held that just war must be declared by legitimate authority, pursued for a just cause once peaceful means had failed, and conducted with restraint and good intent. His reflections were nearly forgotten after the fall of the Western Roman Empire in 476.
Tripartite world
From the ruins of the Western empire, new Christian kingdoms emerged, largely ruled by Germanic warlords. Among this aristocracy, martial prowess and comradeship were core values. Clergy often praised their violence in pursuit of patronage, though the Church still deemed killing sinful and required penance—typically fasting—for absolution.Meanwhile, the Eastern Roman Empire endured, though much of its territory, including Jerusalem, was conquered by the rapidly expanding Islamic Caliphate by the mid-7thcentury. Islam's holiest text, the Quran, addresses —struggle to spread and defend the faith. In the early 8thcentury, Muslim forces entered Europe, conquering much of the Iberian Peninsula. Christians under Muslim rule had to pay a special tax, the. As conquests stabilized, a threefold civilisational order emerged: a fragmented Western Europe, a weakened Byzantium, and an ascendant Islamic world.
Holy wars and piety
Resistance to Muslim advance led to the creation of the small Kingdom of Asturias in north-western Iberia. Over time, this resistance evolved into an expansionist movement, regarded by locals as divinely sanctioned. In the 9thcentury, repeated invasions by non-Christian groups across Western Europe revived the notion of holy war: conflict authorized by a spiritual leader, pursued for religious aims, and rewarded with salvation. Leo IV was the first pope to promise salvation in 846 to those defending the papal territories.As warfare became constant, a new military class of mounted warriors emerged. Known as in contemporary texts, they specialized in weapons like the heavy lance. To restrain their violence, church leaders launched the Peace of God movement. Ironically, efforts to curb bloodshed militarized the Church, as bishops increasingly raised armies to enforce the Peace.
File:Rotunda, Adomanan de locis santis.jpg|right|thumb|alt=A page from a manuscript depicting the plan of a church with lines and a series of five concentring rings|Plan of the Church of the Holy Sepulchre in an early 9th-century manuscript of , a work by the Irish monk Adomnán
With weak central authority, regional strongmen seized control of parishes and abbeys, often appointing unfit clergy. Believers feared such irregularities invalidated sacraments, heightening anxiety over damnation. Sinners were expected to confess and perform penance to be reconciled with the Church. Since penance could be burdensome, priests began offering indulgences—commuting penance into acts like almsgiving or pilgrimage. Penitential journeys to Palestine, known as the Holy Land, held special value, as the region was associated with Jesus's ministry and contained the Church of the Holy Sepulchre, believed to mark his crucifixion and resurrection.
Church reforms
In an age of endemic violence, concern over damnation intensified, fostering reform movements within the Church, which was regarded as the channel through which divine grace was dispensed. In 910, Cluny Abbey's foundation charter set a precedent by granting monks the right to freely elect their abbot. The Cluniac Reform spread rapidly, backed by aristocrats who valued the monks' prayers for their souls. Cluniac houses answered solely to papal authority.The popes, viewed as the successors of Peter the Apostle, claimed supremacy over the Church, citing Jesus's praise for his apostle. In reality, Roman noble families controlled the papacy until Emperor HenryIII entered Rome in 1053. He appointed clerics who launched the Gregorian Reform for the "liberty of the church", banning simony—the sale of church offices—and giving cardinals, senior clergy, the sole right to elect the pope. Andrew Latham, a scholar of international relations, argues that the Gregorian Reform placed the Western Church in conflict with "a range of social forces within and beyond Christendom". By then, divisions in theology and liturgy between Western and Eastern mainstream Christianity had deepened, leading to mutual excommunications in 1054 and the eventual split between the western Roman Catholic and eastern Orthodox Churches.
A spiritual revival took root as new monastic communities like the Carthusians and Cistercians emerged and the Rule of Saint Augustine spread among secular clergy. Christocentrism—a renewed focus on Christ's life and sufferings—also shaped the period, inspiring itinerant preachers.
Prelude to the First Crusade
Four major powers dominated the Mediterranean : the Umayyads in Al-Andalus, the Fatimids in Egypt, the Abbasids in the Middle East, and the Byzantine Empire in southeastern Europe and Anatolia. Within decades, all experienced serious crises, particularly in the east, where recurring droughts and cold waves triggered famine and instability. In contrast, climate change benefitted Western Europe, fuelling economic and population growth.Weakened by internal conflict, Al-Andalus fractured into small states, vulnerable to Christian expansion—a process called the. In Egypt and Palestine, repeated failure of the Nile's floods led to famine and interreligious tension. In 1009, the Fatimid caliph Al-Hakim ordered the destruction of the Holy Sepulchre, though it was later rebuilt with Byzantine support. Meanwhile, Turkoman migrations from Central Asia destabilized the Middle East. The Turkoman chief, of the Seljuk clan, seized Baghdad in 1055; his successor, Alp Arslan, defeated the Byzantines at Manzikert in 1072, opening Anatolia to Turkoman settlement.
As traditional powers declined, Italian merchants gained control of Mediterranean trade. The Normans, originating in northern France, conquered southern Italy and Sicily by 1091. Their expansion threatened papal interests, prompting Pope Leo IX to launch a military campaign against them. Although his campaign failed, he had promised absolution to its participants—a sign of the reform papacy's willingness to invoke spiritual incentives for warfare.
For Western warriors, warfare offered a path to land and power. These ambitions often aligned with reformist popes, who granted absolution to those fighting Muslim powers in Sicily and Iberia. As these territories were once Christian, papal attention soon turned to Palestine. Pope Gregory VII proposed a campaign to reclaim Jerusalem in 1074, though it never materialized. Two years later, disputes over papal and royal authority ignited the Investiture Controversy, reviving interest in just war theory. Anselm of Lucca, a canon lawyer, compiled Augustine's writings to argue that war aimed at preventing sin could be an act of love. The theologian Bonizo of Sutri considered those who died in such wars martyrs. These ideas shaped the belief that just warfare could serve as penance.