Byzantium in the Crusading movement

The Byzantines were active participants in the crusading movement from its inception in the late 11th century until the Byzantine Empire's collapse in the mid-15th century, acting at times as initiators, allies, or adversaries. As heirs of the Roman Empire, they regarded their state as the centre of the civilised world, ruled by a divinely appointed emperor. During the early Muslim conquests the empire lost extensive territories, though from the 960s some lands, including the city of Antioch, were temporarily recovered. Although relations with Western Christianity were strained after the Schism of 1054, Emperor Alexios I Komnenos appealed to the papacy for aid as Byzantine frontiers faced pressure from Muslim Turkomans. Pope Urban II responded by proclaiming the First Crusade at the Council of Clermont in 1095.

Cooperation between the crusaders and Byzantium proved difficult. Although the crusaders initially helped reconquer territories in Anatolia under agreements with Alexios, he prioritised defending Byzantine lands. Viewing this as a breach, the crusaders established new states in the East—Edessa, Antioch, and Jerusalem—by 1100. Despite defeating Bohemond I of Antioch's anti-Byzantine campaign in 1108, Byzantine attempts to seize Antioch failed. The fourth Crusader state, Tripoli, was established south of Antioch with Byzantine support. Emperors John II Komnenos and Manuel I Komnenos compelled the princes of Antioch to swear fealty and attempted to install an Orthodox patriarch in the city. Byzantine influence in the Crusader states reached its height under Manuel, who also secured an oath of fealty from Amalric of Jerusalem, though joint Byzantine–Jerusalemite attempts to conquer Egypt ultimately failed.

From the outset, crusaders often blamed their failures on the "schismatic" Byzantines' lack of cooperation. In the 1180s treaties between the Byzantines and the Muslim ruler Saladin prompted western accusations that Byzantium supported his attempt to conquer the Crusader states. Preparing a crusade, Henry VI, Holy Roman Emperor, forced Emperor Alexios III Komnenos to levy the extraordinary tax known as the Alamanikon. In 1203 the Fourth Crusade was diverted to Constantinople by the Byzantine pretender Alexios IV Komnenos. His overthrow provoked the Sack of Constantinople and the establishment of the Latin Empire with a Catholic patriarch. Greek resistance was led by Byzantine successor states, first Epirus and from the 1230s Nicaea. Despite papal grants of crusade indulgences to those aiding the Latins, the Nicaeans eventually recovered Constantinople under Michael VIII Palaiologos in 1261.