For many people nowadays any war, particularly a war that is not
obviously defensive in nature, appears incompatible with Christianity. Yet this was
not the case at the end of the eleventh century when the First Crusade was launched. Although Christianity never had a concept similar to Islamic jihad that justified and encouraged wars of aggression and conversion, St. Augustine had articulated the concept of ‘just war’ in the early fifth century. This theory that provided the ideological underpinning for the crusades. The summary below is an excerpt from "The Holy Land in the Era of the Crusades: Kingdoms at the Crossroads of Civilizations."
The theory of "Just War" was a theological defense of wars declared by Christian leaders to oppose aggression and oppression. St. Augustine was explicit in condemning wars of religious conversion and also prohibited the use of ‘excessive force,’ but that was the theory. In practice, medieval Christians viewed wars against pagans as legitimate wars. This included the wars against the Vikings in Britain and Ireland, the wars of Charlemagne against the Saxons, and, of course, the war of the Visigoths against the ‘Moors’ (Muslims) in Spain. All these wars were perceived as ‘defensive’ and specifically as a defense of Christendom. Thus, by the eleventh century, Western Europe had a tradition that honored, glorified and even sanctified Christian fighting men, who fought non-Christians.
When Pope Urban II, in response to the request from Emperor Alexis, appealed to knights under his jurisdiction (i.e. knights of the Church of Rome) to go east to liberate Jerusalem, he built upon these traditions. His appeal stressed the fundamental elements of just war (fighting oppression and aggression) by drawing attention to the suffering of fellow-Christians in the Muslim-occupied Near East and by stressing the threat posed by the pagan Seljuks to the New Rome, Constantinople. Yet, Pope Urban expanded on this familiar theme by adding to his appeal the need to liberate Jerusalem.
In contrast to Jerusalem’s peripheral place Islam, Jerusalem was at the very center of Christianity. Islamic scholars might debate about theoretical spiritual ties to Jerusalem, yet it is certain that Mohammed never set foot there. Jesus, on the other hand, had lived and died there. More important, the defining event of Christianity, Christ’s resurrection, occurred in Jerusalem. While the Muslims had Mecca and Medina as their primary and secondary holy sites, for Christians (and Jews) Jerusalem was the unquestioned central and paramount holy site of their respective religions. Period.
It was undoubtedly to inspire men to undertake such an enormously dangerous operation across such vast distances that Pope Urban introduced a startling innovation. He offered spiritual rewards to those who undertook to free Jerusalem from Muslim rule. Contrary to popular myth, Urban did not promise the remission of all sins — certainly not for ‘killing Muslims.’ Nor did he sanction genocide or forced conversions. On the contrary, church documents explicitly state that participation in an armed expedition to liberate Jerusalem would replace already assigned penance for confessed sins. Furthermore, the church carefully conferred benefits only on those who undertook the armed pilgrimage out of piety — but not on those who sought honor or wealth. Yet regardless of what the theologians thought they were offering, many people undoubtedly believed that the armed expedition to Jerusalem would bring them spiritual salvation.