The Crusaders' motivation was by no means fed solely by religious zeal; rather, there were other reasons for their actions, which, moreover, changed over time. The individual motives were:
Religious motives
Building on Pope Urban II's call for crusades at the Synod of Clermont in 1095 (accompanied by the cry "Deus lo vult" - God wills it), many crusaders were convinced that by expelling the Muslims from the Holy Land they were fulfilling God's will and obtaining the remission of all their sins (indulgence, treasure of grace). This must be seen against the background of Christian reports and rumors of atrocities committed by Islamic rulers against the Christian population of the Holy Land, and the devastation of Christian sites, for example the Church of the Holy Sepulchre in Jerusalem in 1009. The Muslim chronicler al-Azimi, who came from Aleppo, also reports Muslim attacks on pilgrims, making access to the holy sites impossible. The preacher Peter the Hermit had also been mistreated by the Turks on an earlier pilgrimage to Jerusalem and forced to repent. In competition with economic interests, religious motives sometimes receded into the background over time - this is particularly evident in the conquest and sacking of the Christian city of Constantinople in the Fourth Crusade. With regard to the crusades to the Orient, however, they never completely disappeared, they also had a great influence on the Christian population in Europe.
Relationship with Islam
A major foreign policy problem for the Christian world was posed by Islam, which in its westward striving initially attacked the Christian Byzantine Empire in the middle of the 7th century. Within a few years, Ostrom/Byzantium lost the provinces of Syria and Egypt, which had been in religious opposition to the Greek and Latin empires since the Monophysite schism, to the Arabs, who were perhaps welcomed there as liberators by parts of the population (this is disputed in research); it continued, however, to assert Greek-influenced Asia Minor. Western North Africa resisted the Arabs until the end of the 7th century, while the Spanish Visigothic Empire collapsed within a few months under Arab storm around 700, so that the Arabs in the west were only stopped and pushed back by the Frankish Empire.
After the Byzantine Empire had been pushed out of central Italy by the Lombards in 751 (fall of the Exarchate of Ravenna), it was mainly limited to the Orthodox heartland of Asia Minor, the coasts of the Balkans and southern Italy at the beginning of the 8th century. Subsequently, in the 9th and 10th centuries, the empire found a modus vivendi with the Arabs, which even resulted in military alliances with individual Arab states. The military revival around the year 1000 was followed by an internal decline. With the Islamic Turkic people of the Seljuks, a new, expansive power entered the political stage of the Middle East at the same time, expanding at the expense of the Arabs and Byzantines. This led to military disaster for the Byzantines in 1071 in the Battle of Manzikert against the Seljuks, marking the beginning of the Turkish land grab in Anatolia.
In 1085, the Byzantine emperor Alexios IKomnenos, having repelled the Norman invasion of Epiros and Macedonia (with the aim of conquering Constantinople), finally left Asia Minor completely to the Seljuks in return for an oath of fealty, except for a few bases, so as not to be worn down between two opponents. After the victory over the Normans, Alexios asked the Pope for support to reconquer the Asia Minor empire, which by then had fragmented into several Turkish emirates, which Byzantine diplomacy played off against each other.
The great military effort of all Christian powers of the time can be explained by the fact that Islam was seen as a great danger - not only for the Byzantine Empire. After all, the Islamic-Arab area of power bordered on France at the Pyrenees; moreover, almost all the Mediterranean islands and parts of southern Italy had been temporarily conquered by Arabs. The latter were repeatedly attacked by them even after reconquest. Byzantine Sicily was conquered by the Arabs from 827, then by the Normans, until it fell to Henry VI in 1194, whereby the empire of the Hohenstaufen also directly bordered the Islamic sphere of power.
Relationship with Orthodoxy
The Oriental schism of 1054 strained the relationship between Orthodox and Catholic Christians from the beginning of the Crusades. Another aspect is the political relationship of the two leading powers of the Catholic and Orthodox world of states, respectively. The proper name of the German as well as the Byzantine empire was "Roman Empire", and the respective emperor derived from it a claim to leadership over the entire Christian world of states. Byzantium pursued an expansive western policy in the 12th century. Dynastic marriages with the Hungarian and German ruling dynasties, as well as military interventions in Italy with the aim of also gaining the (western) Roman imperial crown, were a basic constant of the foreign policy of the Byzantine Comnene dynasty. In order to push back the influence of Venice in the Byzantine Empire, Constantinople pursued a sharp anti-Venetian policy in the second half of the 12th century. This, of course, was not without reaction in Western Europe. The crusades were therefore increasingly directed not only against Islam, but at the same time more and more against the Orthodox, Greek-influenced Byzantium.
Nevertheless, the religiously motivated idea of crusading remained a recurring component of European politics in the period that followed, even if scholars sometimes emphasize that the crusading idea lost some of its force from the thirteenth century onwards (see the section on controversies in historiography above). On the whole, its importance in the late Middle Ages may probably not be overstated. Thus, a military expedition was considered in 1453 to defend Constantinople against Sultan Mehmed II. However, this half-hearted expedition started rather late, in April 1453, whereas the Sultan had already begun constructional preparations for a possible siege in the spring of 1452 and made no secret of this.
Whether one may place the concerted military assistance of Christian powers, such as the Holy Roman Empire and Poland, in the defence of Vienna in 1683 against the Turks in the crusading tradition is questionable. In 1528, in fact, an event occurred that had been unimaginable only a few decades earlier: France and the Ottoman Empire formed an alliance against the Habsburg Empire. At the latest with the integration of the Muslim state into the alliance system of the Christian powers, the unifying claim of the Catholic crusade idea in European politics ended.
Social factors in Europe
The occidental nobility hoped for new possessions through the conquest. This also and especially applied to the younger sons of the nobility, who were not entitled to inherit and now saw the chance to be able to rule over their own territory after all. This was also a goal of the church, since the peace of God was disturbed again and again by conflicts, in which it was primarily about territorial disputes. Thus, the crusades also provided a welcome occupation for the surplus sons who could not or did not want to be placed in the monastery or the clergy.
Large parts of the rural population saw the crusade as an escape from the harsh and often very unjust living conditions at home - especially since the Pope had promised an end to serfdom for anyone who would take up the cross and go with them to the Holy Land. The crusaders were joined in the procession by non-combatants: Women, clergy, the elderly and the poor.
Criminals and outlaws also responded to the calls because their crusading vows enabled them to escape prosecution and they hoped for a new life or booty.
Economic policy motives
Economically, the Italian maritime republics (Genoa, Pisa, Venice and others) also benefited from trade with the Orient. Thus, it was briefly considered to conduct a crusade to secure the spice route. However, the idea was dropped again quite soon.
The papacy expected a massive strengthening of its power position from control over the Holy Land. Ultimately, the popes probably also hoped for reunification with or control over the Eastern Church. In addition, with the beginning of the Fourth Crusade, economic interests also dominated. The best example of this motive is probably the Fourth Crusade itself, which was diverted from the commercial metropolis of Venice to Constantinople and resulted in plunder by the crusader army with the booty being transported to Venice in order to eliminate the trade rival. This shows the complete perversion of the originally religious crusade idea on the one hand, and on the other hand also a reason for the ever diminishing effect of the crusades in the defense of the Eastern Roman Empire.
The crusades were financed in the individual bishoprics through the crusade tithe. For this purpose, official books such as the Liber decimationis were created.
Other factors
The British historian Robert Bartlett sees the Crusades in a larger, pan-European context: In the 11th century, a strong population growth sets in, due to favorable climatic circumstances and new developments in agricultural technology. The population surplus leads to an expansion into the peripheries of Europe: the Iberian Peninsula, Ireland, Germania Slavica, the Baltic States and also into the Holy Land.
