The Byzantine Empire participated in the crusading movement from its inception in the late 11th century, serving as initiator, ally, or adversary. The Byzantines regarded their state as the continuation of the Roman Empire and the centre of the civilised world, although it had lost substantial territories during the early Muslim conquests. The empire recovered some territory after , but from the mid-11thcentury faced sustained pressure from several directions, including nomadic Turkomans in Anatolia.
Despite strained relations with Western Christendom after the East–West Schism (1054), the Byzantine emperor Alexios I Komnenos sought Western aid, prompting Pope Urban II to proclaim the First Crusade in 1095. Cooperation proved difficult: although the crusaders helped recover parts of Anatolia, they founded four states on former Byzantine lands and often blamed the "schismatic" Byzantines for later failures. Byzantine influence in the Crusader states nevertheless grew as Muslim power in Syria expanded under the Zengids and later Saladin; Manuel I Komnenos extended Byzantine suzerainty over the Principality of Antioch and the Kingdom of Jerusalem and supported joint campaigns against Egypt.
Byzantine actions against Westerners and negotiations with Saladin increased Western resentment. Although plans for a crusader attack on Constantinople had long existed, the city did not fall until the Fourth Crusade, largely due to internal Byzantine conflicts. The Sack of Constantinople and the establishment of the Latin Empire on former Byzantine territory in 1204 deepened the rift between Western and Eastern Christians. Byzantine successor states, particularly Nicaea, led the resistance to Latin rule and restored the empire under Michael VIII Palaiologos in 1261. Later Western plans to reconquer Constantinople, though supported by the papacy, had failed by 1320.
Late Byzantine rulers sought Western aid against Ottoman expansion through alliances, papal negotiations, and church union, but civil wars, anti-Latin sentiment, and Ottoman military successes repeatedly frustrated crusading plans. Western expeditions brought only limited relief before the Fall of Constantinople in 1453. Mutual suspicion between Byzantines and crusaders shaped contemporary narratives and later historiography, while the Fourth Crusade left enduring cultural trauma and a mixed artistic and literary legacy.
Background
The Byzantine Empire was, as the historian Peter Lock observes, "in every sense the heir and embodiment of the Roman Empire", which had been divided into eastern and western halves in 395. The eastern emperors ruled from Constantinople, refounded in 330 by the first Christian emperor, Constantine the Great, on the site of Byzantium. After the Fall of the Western Roman Empire in 476, Byzantine rulers expressed their claim to universal authority through the concept of and the title "Emperor and Autocrat of the Romans". Byzantines called themselves ('Romans') and associated their empire with the , or the civilized world, regarding it as something more than a merely temporal realm.
The emperors were widely regarded as God's representatives on earth. The Church was organised around five major episcopal sees, each headed by a patriarch. Among them, the bishops of Rome, styled popes, enjoyed honorary primacy but were expected to act as imperial officials. The Byzantine state was administered by an educated bureaucratic elite; many were eunuchs, who were ineligible to the imperial throne. In foreign affairs, Byzantium preferred diplomacy over warfare, using wealth, titles, and elaborate ceremonies to manage rivals.
Byzantine power declined under pressure from enemies, including the Sasanians and Avars. From the 630s, the empire proved unable to resist the expansion of the first Islamic Caliphate and lost the Middle East, Egypt, and North Africa. Islam arose under Muhammad, whose revelations formed the Quran, regarded as God's final message. Early Muslim expansion was encouraged by Quranic promises of afterlife reward for participation in , or holy war. As a result, the patriarchates of Antioch, Alexandria, and Jerusalem came under Muslim rule, leaving only Rome and Constantinople under Christian control. Muslim rulers classified Christians as , protected yet legally subordinate.
In Western Europe, former Roman territories fragmented into smaller kingdoms ruled largely by Germanic elites. Under the Carolingians, the Franks reunited vast regions that had been part of the Western Roman Empire. Cooperation between the papacy and the Carolingians grew amid tensions with Byzantium, including disputes over Byzantine iconoclasm. In 800, Pope Leo III crowned the Frankish king Charlemagne emperor. Although the iconoclasm ended, Western adoption of the clause in the Nicene Creed deepened divisions with Eastern Christianity. By the mid-ninthcentury, the Carolingian Empire had divided into smaller, decentralised states, including France and Germany. A renewed imperial project emerged under the German king Otto I, crowned emperor in 962, but the Holy Roman Empire, encompassing Germany and much of Italy, remained politically fractured. The diffusion of authority in the West, amid endemic violence and instability, contributed to the rise of a mounted warrior elite later known as knights. This Western elite produced large numbers of younger, landless men-at-arms seeking to exploit the unsettled conditions prevailing along the borderlands of Catholic Europe.
After 963, the Byzantine Empire entered a renewed phase of expansion as the weakening of the Abbasid Caliphate enabled the recovery of the islands of Crete (961) and Cyprus (965) and the cities of Antioch (969) and Edessa (1031). Although emperors sought to present their campaigns against Muslim powers as holy wars, the concept remained alien to Byzantine theology. From 1027, treaties with the Fatimid Caliphate—the Abbasids' rivals—recognised the emperors as protectors of Christians and confirmed their right to appoint the patriarchs of Jerusalem. Military success encouraged economic growth in the empire, while Constantinople prospered through its position on major trade routes. With a population of more than 350,000, it ranked among the world's largest cities. Commercial contacts with Western Europe revived, largely through merchants from the Italian city-states, notably Venice, Pisa, and Genoa.
Prelude to the First Crusade
At the death of the conquering emperor Basil II in 1025, Byzantium dominated south-eastern Europe and the Middle East; yet within fiftyyears, it faced mounting threats. Explanations for this decline include weakened central authority, institutional shortcomings, and intellectual stagnation. Meanwhile, cold spells across the Eurasian steppes displaced Turkic groups: the Pechenegs crossed the Danube, while Seljuk-led Turkomans established control over the Abbasid Caliphate and raided Anatolia. They also seized Antioch and Edessa.
From the 1040s, Norman warlords attacked papal and Byzantine territories in Italy, prompting Pope Leo IX to seek an alliance despite ecclesiastical tensions. Yet, in 1054, disputes over liturgical practice and Byzantine concerns about the encroachment of Western customs upon the Greeks of southern Italy culminated in mutual excommunications. This rupture coincided with the Gregorian Reform, which strengthened papal authority and, during the Investiture Controversy, fostered Western ideas of just war. Thereafter, Western Christians conceived the —the Christian world—as divided between two empires, one under the Western "Emperor of the Romans" and the other under the Eastern "Emperor of the Greeks", but united under the papacy.
Following the Schism of 1054, Byzantine–papal relations fluctuated. Although Pope Stephen IX sought reconciliation, NicholasII allied with the Normans. A Byzantine embassy to Rome in 1062 reportedly mentioned an expedition to Jerusalem. Meanwhile, Turkoman pressure intensified, culminating in a Byzantine defeat by the Seljuk sultan Alp Arslan at the Battle of Manzikert in 1071. The ensuing civil war facilitated Turkoman settlement in Anatolia. Renewed contacts with the West included dynastic ties and Pope Gregory VII's plan for a papally led expedition, but this was disrupted by the Investiture Controversy.
In 1081, Alexios I Komnenos seized the throne. He secured Venetian naval support against the Normans through commercial concessions and also restored control over the Danube frontier. By then, Anatolia had fragmented into rival Turkoman polities, such as the Sultanate of Rum and the Danishmendids. Although initially excommunicated by GregoryVII, Alexios maintained diplomatic relations with Western powers because Western mercenaries were central to Byzantine armies. To forestall a Byzantine alliance with his opponents, the new pope, UrbanII, absolved Alexios of the excommunication. In 1095, Alexios sent envoys to Urban at the Council of Piacenza to seek military assistance. At that time, as the Byzantinist Ralph-Johannes Lilie observes, the papacy was the only Western power with "a vital interest" in assisting Byzantium.
First Crusade
Council of Clermont
Shortly after the Council of Piacenza, Pope UrbanII travelled to France on a preaching campaign culminating in the proclamation of the First Crusade at the Council of Clermont on 27November 1095. He presented the expedition as an armed act of penance, promising spiritual rewards to ease knights' troubled consciences and prompting many to "take the cross", sewing cloth crosses onto their robes. Although Urban stressed the defence of eastern Christians, for most participants the liberation of Jerusalem became the chief aim.
Emperor AlexiosI likely expected a controllable force, but Urban's appeal aroused unexpected enthusiasm, which led to the formation of several armies totalling over 30,000armed men and perhaps 70,000non-combatants. Urban set 15August 1096 as the departure date, and instructed the crusaders to assemble at Constantinople. Given little time to prepare, Byzantine authorities were alarmed by the large forces beyond imperial control. They adopted what the Byzantinist Jonathan Harris calls a "carrot and stick" strategy: escorting the crusaders with imperial troops while supplying provisions. As most came from lands formerly within the Carolingian Empire, the Byzantines called the crusaders ('Franks').
People's Crusade
Meanwhile, Peter the Hermit, a charismatic preacher, organised a recruitment movement in northern France and eastern Germany. By March 1096, he had gathered over 10,000largely ill-equipped followers from varied social backgrounds. The expedition travelled in two contingents. The first, led by the knight Walter Sans Avoir, reached Constantinople without serious incident. The second, led by Peter, clashed with Byzantine forces at Niš over supplies before arriving on 1August. The crusaders' plundering of the suburbs prompted Alexios to move them into Anatolia. They were almost entirely destroyed by Kilij Arslan I, Sultan of Rum, at the Battle of Civetot on 21October 1096.
Princes' Crusade
The official crusading armies entered Byzantine territory from several directions between October 1096 and April 1097. Hugh of Vermandois was blown off course near Dyrrachium and escorted to Constantinople. Other armies, led by Bohemond of Taranto, Godfrey of Bouillon, and Raymond IV of Toulouse, were guided by Pecheneg mercenaries. Byzantine authorities intercepted communications between the crusading armies and required them to camp outside the city. Skirmishes between crusaders and Byzantine troops were frequent, and Bohemond even proposed attacking Constantinople. Emperor Alexios required the Frankish leaders to swear oaths to restore former Byzantine territories. Though some resisted, most complied.