Crusading movement
Framework of Christian holy war
The Crusading movement was a major religious, political, and military endeavour of the Middle Ages, generally dated from the Council of Clermont (1095), at which Pope Urban II proclaimed the First Crusade, an armed expedition in support of Eastern Christians under Muslim rule. He framed it as a form of penitential pilgrimage. By this point, 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, whose defence inspired successive Crusades. The papacy also launched crusading campaigns against other targets—Muslims in Iberia, paganism in the Baltics, 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 (absolution from sins), material gain also played a part. Crusading campaigns were typically initiated through papal bulls, and participants pledged 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 strong 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 of which survived 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 18th century.
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; defence, restitution, and punishment were considered acceptable grounds.
Although the Bible—Christianity's core scripture—presents conflicting views on violence, the 4th-century Christianisation of the Roman Empire led to the development of Christian just war theory. Bishop Ambrose was the first theologian to equate enemies of the Christian state with those of the Church.
In 395, the Roman Empire was permanently divided into eastern and western halves. 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 (Byzantine) Empire endured, though much of its territory, including Jerusalem, was conquered by the rapidly expanding Islamic Caliphate by the mid-7th century. Islam's holiest text, the Quran, addresses jihad—struggle to spread and defend the faith. In the early 8th century, Muslim forces entered Europe, conquering much of the Iberian Peninsula. Christians under Muslim rule had to pay a special tax, the jizya. 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 9th century, 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 milites 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.
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 Henry III 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 c. 1000: the Umayyads in Al-Andalus (Muslim Spain), the Fatimids in Egypt, the Abbasids (nominally) 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. Climate change benefitted Western Europe, fuelling economic and population growth. Western cities remained comparatively small: even the largest, such as Venice and Rome, had fewer than 40,000 inhabitants.
Weakened by internal conflict, Al-Andalus fractured into small states, vulnerable to Christian expansion—a process called the Reconquista. 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. Their chief Tughril I, of the Seljuk clan, seized control of the Abbasid Caliphate 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 papacy's willingness to invoke spiritual incentives for warfare.
Western knights' desire for land and power aligned with increasingly assertive popes who granted absolution for campaigns against 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. Soon, disputes over papal and royal authority ignited the Investiture Controversy, during which armed conflict renewed interest in theories of just war. 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.
Crusading campaigns
Revived interest in Augustine's teaching on legitimate violence provided the Western Church with an ideological framework for military engagement. By the late 11th century, amid heightened concern over sin, the papacy was well positioned to mobilize the warrior class's values.
First Crusade
Facing Turkoman incursions, the Byzantine emperor Alexios I Komnenos sought military aid from Pope Urban II in 1095. Seeing this as a chance to reassert papal authority, Urban called for a campaign against the Turkomans at the Council of Clermont, offering spiritual rewards to participants. The historian Jonathan Riley-Smith views this as a "revolutionary appeal" that linked warfare to pilgrimage.
Urban's appeal sparked unexpected enthusiasm. In early 1096, more than 20,000 poorly organized crusaders set off in what became the People's Crusade. Most perished or were massacred en route. A second wave, better coordinated, followed between August and October in that year, comprising at least 30,000 warriors and as many non-combatants, led by prominent aristocrats including Raymond of Saint-Gilles, Bohemond of Taranto, and Godfrey of Bouillon. They advanced through fragmented Muslim-held territories and captured the cities of Edessa, Antioch, and Jerusalem by July 1099.
Crusades for the Holy Land
The first crusaders consolidated their conquests into four Crusader states: Edessa, Antioch, Jerusalem, and Tripoli. Their defence prompted new campaigns, the first as early as 1101. Several expeditions, especially those led by monarchs, are distinguished by numerical labels. These campaigns brought about near-continuous warfare in the region, drawing forces from across the wider world, including crusaders from Western Europe, slave soldiers from sub-Saharan Africa, and nomadic horsemen from the Eurasian steppes.
Edessa's fall in 1144 to the Turkoman leader Zengi triggered the Second Crusade, led by Louis VII of France and Conrad III of Germany, which failed in 1148. Zengi's son, Nur al-Din, unified Muslim Syria and dismantled the Fatimid Caliphate. These lands came under the control of Saladin, an ambitious Kurdish general. In 1187, he destroyed the Jerusalemite field army at Hattin and captured most Crusader territory, including the city of Jerusalem.
The resulting crisis triggered the Third Crusade, led by Emperor Frederick I, Richard I of England, and Philip II of France. Although Jerusalem remained under Muslim rule, the Crusader states endured, and the Kingdom of Cyprus was founded on former Byzantine territory. Later Crusades focused on recovering Jerusalem, but the Fourth was diverted by the Byzantine claimant, Alexios Angelos, leading to the sack of Constantinople and the creation of a Latin Empire in the Aegean in 1204. The Fifth Crusade against Egypt failed in 1217–21. The Sixth regained Jerusalem in 1229 through negotiations by the excommunicated Emperor Frederick II, but the city was sacked in 1244 by Khwarazmian raiders. Its loss prompted Louis IX of France to launch the Seventh Crusade in 1248, which ended in defeat.
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