The Crusades were a specific form of Holy War initiated by the pope. The canonical legitimacy of the Crusades was constituted by the papal convocation, the papal indulgences, and the vows taken by the participating knights. For the Syr. Christians the Crusades at the time had simultaneously positive and negative effects. In the long term the Crusades enhanced religious polarization and undermined the position of Christians in Muslim countries.

At the Council of Piacenza, in March 1095, the Byzantine Emperor Alexios I Komnenos (1081–1118) requested Latin military aid against the expanding Muslim forces. With a view of restoring the union of the church and claiming for himself universal ecclesiastical authority, Pope Urban II (1088–99) began to campaign for a war aimed at liberating the Holy Places. He appealed to already existing conceptions of Holy War and developed them into a theory of an armed pilgrimage. At the Council of Clermont-Ferrand, in Nov. 1095, his idea of fighting a war for God met with immense religious enthusiasm. When the knights set out for Jerusalem in 1096, they were joined by pilgrims and ardent non-combatants of different origins. Pope Urban had ignited a formidable movement, which was difficult to control and which resulted in fundamental political, cultural, and economical changes in Europe and the Middle East.

Several vast military campaigns ensued, which were often led by the most powerful crowned heads of Europe. The Crusade of 1096 being the first, the next ones are traditionally counted as the Second Crusade (1145–1149), the Third Crusade (1187–92), the Fourth Crusade (1198–1204), the so-called Children’s Crusade of 1212, the Fifth Crusade (1217–1221), the Crusade of Frederick II (1228–1229) and the Crusades of King Louis  IX the Pious of France (1248–54, 1267–70). In addition, many more military campaigns with papal authorization were waged against Muslims between the 12th and the 15th cent. None of these, however, succeeded in bringing the expansion of the Turks to a halt, nor did they prevent the conquest of Constantinople in 1453. In Europe Crusades were waged against pagan peoples, against heretical movements, as well as against political enemies of the pope. The Reconquista on the Iberian Peninsula also became part of the Crusade movement. Even though criticism flared up against individual wars, the concept of Crusades as well as the ideal of the virtuous knightly Crusader remained strong.

Jerusalem was first captured in the year 1099. Against the wishes of the Byzantine emperor, who had expected the return of the re-conquered regions to his own rule, Latin Crusader States were established in the Levant as a result of the First Crusade (Kingdom of Jerusalem, County of Tripoli, Principality of Antioch, County of Edessa). The ruling French and Norman families by and large implemented the legal traditions of their homelands. Within the Crusader States all non-Latins were legally subordinated. On the local level traditional forms of autonomy and jurisdiction remained intact. The descendents of the first Crusaders developed a new Middle-Eastern identity and specific forms of relations to, as well as agreements with, Eastern Christians and Muslims. These Crusader States, although much reduced toward the coast, were sustained until the year 1291, which saw the fall of Acre.

As for the Christians, the discriminating poll tax under Muslim rule was lifted. The Greek Orthodox hierarchy was forced under the supremacy of a new Latin hierarchy built up in the Crusader states. Local Christians of the Greek Orthodox denomination are known as the ‘Suriani’ in the Latin sources. They made up a considerable part of the Christian population all over the Levant. Albeit speaking Arabic in daily use, they themselves cherished their Syrian or Aramean origin. Their liturgy was byzantinized during the Crusader period.

As the Syr. Orth. were considered heretical by the Latin Church, their hierarchy as well as the hierarchy of the Maronites on the whole was left intact. Almost all the areas of Maronite population fell under Latin rule in the County of Tripoli. Their patriarch resided in monasteries in the Maronite diocese of Jubail. Due to an extreme scarcity of sources, the history of the Maronites in this period is much disputed. The Latin chronicler William of Tyre regarded them as fine warriors and as allies of the Latins.

The Syr. Orth. archdioceses of Edessa, Samosata, Manbij, Tarsus, Antioch, and Jerusalem, as well as important bishoprics such as Acre, Tripoli, Cyprus, Marʿash, Raban, Kesoun, and Saruj became part of the Crusader states. In the County of Edessa the Syr. Orth. became involved in the Latin administration and also in military action. The capture of Edessa by the Zengid army in 1144 and the failed Latin attempt to regain the city caused much hardship among the Christian population. Massacres and slavery ended a short-lived period of pride and affluence. A spiritual crisis among the Syr. Orth. and theological debates flared up as a consequence.

The Ch. of E. came into contact with the Latins only on its periphery. Under the authority of the metropolitan of the Ch. of E. in Damascus and probably a second metropolitan of Jerusalem and Tripoli, members of this church lived also in Edessa, on Mount Lebanon, in Cyprus, in Antioch, and in the merchant towns on the sea coast. The communities were perhaps also enlarged by refugees, who had moved to the West during the Mongol invasions in Central Asia and Mesopotamia.

The intensity of theological relations between the Eastern Churches and the Latins depended on dogmatic positions as well as on their geographical dispersion and on political constellations. A general climate of theological openness led Eastern Christians and Latins in the 12th and 13th cent. to study each other’s theological propositions and also to seek common ground, with different outcomes. While the union between the Latin and the Greek Church still was officially upheld in the Levant, Latin ecclesiastical supremacy and conflicts alienated the two Churches. During the 12th cent. almost no contacts between the Latins and the Ch. of E. are recorded. At the same time, the most isolated of the Syriac churches during that period, the Maronites, were officially united with the Roman Church. The Maronite patriarch Jeremiah II (1199–1230) was accepted as Primate of the Maronite Church and invited to the Fourth Lateran Council in 1215. Some opposition against Latinization however flared up, most notably towards the end of the Latin rule in the Levant.

The Syr. Orth. Church sought a pragmatic understanding without giving up its independence, maintaining contact with the Latins and with the Byzantine Emperor Manuel. At the council of Jerusalem in 1141 the dogmatic differences between Latins, Armenians, and Syr. Orth. were not felt as grave as previously expected. Joint religious practice as well as theological disputations took place. The Syr. Orth. used the advantages of a government that was often unfavorable to the Greek Orthodox. In turn, several instances of direct church governance by the Latins can be detected in which the Latin authorities overruled internal decisions by the Syr. Orth. patriarchs or synods.

During the 13th cent. the Pope intensified his diplomatic and missionary efforts among the Ch. of E. and the Syr. Orth. Church. The generally tolerant attitude of the Mongols towards Christians raised Latin hopes for new allies against the Muslims. The residences of the mendicants in the Crusader States served as bridgeheads for the missions, while the local Latin Church seems to have been little involved. Individual prelates of the Syr. Orth. Church and the Ch. of E. maintained close relations with the Latins. No Syr. source, however, corroborates Roman claims that some Syrian metropolitans or patriarchs recognized Roman primacy at that time. Such claims could be due to a misunderstanding of the Syrian attitudes. Such unions were in any case ephemeral and not supported by the communities.

The frequent wars, their economical effects on local trade and agriculture, roaming bandits, and encroachments on the Eastern Christians caused hardship. Refugees fled to the coast especially after the final Muslim conquest of Edessa in 1146. A slow deterioration of the relations between the Christian and Muslim population is noticeable. Neither Muslims nor Latins sufficiently secured the indigenous population. At the same time for Syriac Christians this was a period of most important achievements in all fields of art and culture, for which the term ‘Syriac Renaissance’ has been often used (see in particular Kawerau 1960 and Teule et al., 2010).