The Crusades were symptomatic of the alteration in religion which occurred during the late Roman Empire, and the reorientation towards the Abrahamic faith of the people of the former Roman Empire and lands to the east.
Whilst religion had always played a role in warfare (e.g. Mithraism, Mars, Ares etc), religion had not previously completely permeated virtually every detail of everyday life as it did with the decline of the Roman Empire. Suddenly religion, a force which had previously been just an explanation for the world and a place to channel superstitious beliefs or to try and extract favour, became instead a deciding factor in how society structured itself. National or civic identity has previously been the main way to identify oneself prior to this change. Afterwards, it was your religious creed.
Religion became the primary focus for invasion and conquest when Islam took root in Arabia. Muhammad was an intelligent, charismatic and politically talented individual. He realised the opportunity to make use of his newly united Bedouin army against the much weakened Empires of Byzantium and Persia was too good to pass up. While the usual motivators of loot, glory and migration were all present in the expansion of Islam, the principle motivator behind the invasions was religion. This was a peculiar change in warfare. While religion had at times been a contributing factor to waging war (e.g. the recent Persian-Byzantine war), it had not previously been the main reason. Interestingly, jihad typically refers to one's own spiritual struggle within, rather than a strategic military campaign. But from the outset of Islam, borne of the conditions which the Arabs had long lived, jihad did also become a manifestation of spiritual warfare with temporal aims and enemies.
For some reason, this new trend didn't really catch on in Byzantium. Byzantium behaved as a nation state typically did, pragmatic. There were no notions of romantic adventures in the name of religion, although the Emperors were delighted when their campaigns resulted in the recovery of a holy relic or some other achievement of significance which would please the faithful.
In the West, however, the idea of religion as the key motivator and justification for military activity did catch on. In Iberia, the Christian kingdoms came to abandon their policies of relative tolerance and statecraft once the fundamentalist Berber warriors erupted into the Iberian peninsula. This eruption changed the relative tolerance on both sides into a struggle based increasingly on religious ideals.
Across the Pyrenees, the fervour caught on in the most militarily up to date part of Europe: France. The French liked to think crusades were their affair, and quite often crusades were almost entirely French. But others such as Germans, English, Flemish and Italians also took part at times. Some who took part did so for non-religious reasons. The Italian maritime cities did so for desire for trade, Byzantium did so in an effort to gain allies against the enemies of their state. But the Crusades did genuinely represent for many a path to salvation and an expression of their faith through violent military aggression. In an age where you could drop dead at any time and find your sins being judged at the Gates of Heaven, the chance to atone offered sincere hope and motivation to many who participated. While Western Europe was economically and militarily in the ascendent at the time, the Crusades happened as they did largely through the influence of religious institutions.
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