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The Causes and Goals of the Crusades: Religion and Just War Theory

Updated: May 30

Frédéric Schopin - Battle Beneath the Walls of Antioch (June 1098)

When talking about the causes of war, there are those who all too readily unsheathe some phrase similar to “religion causes all wars!” This, of course, is false – and I intend to discuss that point in more detail in a later article. What is of relevance here is that often people will go on to cite the crusades as an example of religious violence, which is why I felt it necessary to take a deeper look into what exactly led up to this fascinating and controversial set of events - what were the causes, motives, and goals of the crusades?

The crusades were a series of military campaigns organised by Christian powers in order to retake the Holy Land back from Muslim control. Each campaign was met with varying degrees of success and failure but, ultimately, the wider objective of keeping Jerusalem under Christian control failed.

Islamic Expansion and the Byzantine Empire

The Battle of Mohacs (1526)

First and foremost, we should talk about Islamic expansion, because without it, there would be no crusades. Islamic forces were organised, aggressive and effective at spreading the faith and spreading it by the sword. They were good students of Muhammad, who was, to say it generally, a warlord. They even earned admiration from a number of the crusaders for their military prowess.

Greco-Roman conflict with Islamic forces started at the very beginnings of Islam, in the 7th century – the crusades were merely a chapter in this history. Literally since its birth, Islam projects power throughout the middle east and Mediterranean (expanding my force was seen as legitimate). Muslims were quite brutal and oppressive toward the infidel, to varying degrees depending on the sect.

To help us grasp the scope of Islamic expansion, please see Bill Warner’s map/timeline comparing the breadth of Jihad and the Crusades:

It must be noted that this was the Middle Ages – violence was everywhere – and while that doesn’t excuse the brutality of those who lived at the time, we should refrain from using a modern lens to interpret the violence of the past. Bull writes that it was a time marked by “a society where violence was endemic and in itself unremarkable” (Origins, 1995).

Unfortunately, it’s true that the Church failed to distance itself from the violent world around it, but we can’t so simply think of the crusades as a violent rush. The Church was known for being a brake on forces of change and, because they had a near-monopoly on the written word, was more systematised in how they dealt out force. The Church wasn’t known for just randomly rushing into things.

From Bill Warner’s map, it should be clear that there’s a very real sense that most of the Crusades were defensive wars and showcase a genuine desire to free their fellow Christians and take back the Holy Land. After 400 years of conflict, Christians begin to push back.

Muslims had taken Jerusalem from Christian powers as early as 636-8 AD and had allowed pilgrimages because they could profit from it. Later, in around 1073 AD, the Seljuk Empire (a different Islamic empire) captured Jerusalem and only then did tensions accelerate, making pilgrimage difficult to say the least. The Seljuks went on to defeat a Byzantine army at the Battle of Manzikert, temporarily capturing Byzantine emperor Romanos IV Diogenes. This led to weak defences and political instability in Byzantium, even upon the emperor’s return. Many Byzantine commanders left their commands to stake their claim for the throne, leading to civil war in Byzantium, and the Seljuk Turks took full advantage of this military neglect, taking control of considerable territory.

Alexios I Komnenos, after seizing the throne, bolstered the garrisons of Constantinople, which weakened both potential threats to his rule and Byzantine military defences. Some political stability had been regained, however fear of the rising power of some of the commanders and military provinces, and doubts about the loyalty of Norman mercenaries, led him to weaken the military. Naturally, this only made the Seljuk threat more prominent. Emperor Alexios I feared the advance of the Seljuk Turks towards his capital city of Constantinople. Seeing the Seljuk control of Jerusalem as a means to tempt European leaders into action, Alexios appealed to west (in 1095 AD) to help recover both the Holy Land and all those parts of the Byzantine Empire which had been conquered.

Pope Urban II and the Council of Clermont

Pope Urban II at the Council of Clermont (1095)

The launch of the crusade presented Pope Urban II with a chance to move closer to the Orthodox church and heal the rift made by the schism in 1054. He hoped to reunite the Latin West with the Greek East, with himself as the head. Additionally, a crusade would increase the prestige of the papacy (having experienced threats from Holy Roman Emperors in the previous century), as it led a combined western army, and it would consolidate the papacy in Italy itself. He didn’t have much interest in gaining land, and the initial oath of the crusader was that the land would go back to Byzantium. It seems that he was more interested in rescuing fellow Christians, uniting the Christendom, and securing the reputation and influence of the papacy.

As the spiritual leader of the western Catholic Church, Urban II may have only been further motivated by the idea that offering up the crusades as a means of redemption (as pilgrimage) would also meet the spiritual needs of the believers under his guidance.

Urban II called for crusade in a speech during the Council of Clermont (1095). He leveraged particularly on the ideas of pilgrimage, holy war, and penance. Bull writes that “… he was able to mobilize the resources, enthusiasm, and communication skills of many individual clerics and religious communities…” (Origins, 1995).

Within an age of such intense religiosity, the city of Jerusalem held a central role. Pilgrimage was understood as a profound act of devotion and penance, but with the Seljuks in control of the Holy Land, Christians could no longer visit key holy sites. Naturally, this provoked a bit of a spiritual crisis. They desired to access shrines associated with the life and ministry of Jesus, particularly the Holy Sepulcher, the church in Jerusalem said to contain the tomb of Christ.

Urban II promoted the “plenary indulgence” – a full pardoning of all the penance one owes for committing sins. It was a promise of eternal glory and absolution from sin. This was an opportunity to bypass purgatory and go straight to heaven. Those who defended Christendom would be embarking on a pilgrimage, all their sins would be washed away. Bull writes that “their unavoidable immersion in worldly concerns meant that it was impossible for them to perform all the time-consuming and socially disruptive penances which could keep pace with their ever-increasing catalogue of misdeeds…. Here at last was a spiritually effective activity designed specifically for lay people” (Origins, 1995). At this time, genuine belief in the concept of sin and damnation was widespread and, because people struggled to keep up their penance, this indulgence would have appealed to many. Additionally, citizens could be released from excommunication.

Urban II saw the campaign as a chance for knights to direct their zeal towards what was seen as a righteous, praiseworthy, and spiritually meritorious act. It was an opportunity for sinful knights to cease their endless in-fighting and exploitation of the weak and make good their violent lives.

They only expected a few thousand to respond to the call, however (largely thanks to Urban’s preaching tour, and independent preachers who advertised it (some churches acted as recruitment centres)), an estimated 90,000 people (probably more) of all classes (from nobles to peasants – but mostly peasants) were persuaded to participate in the First Crusade. They stirred up so much religious fervour, that an unofficial “Peoples Crusade” would march to Constantinople to heed the call in 1096.

A Just War

St Augustine writing Pensamientos de San Agustin

The idea of crusading was new and came as a shock to many people – though this newness may have contributed to its popularity. But naturally, it would need to be justified. The Council of Clermont renewed the idea of the “Peace of God,” responding to a peace movement that had developed around that time. This movement sought to protect those in distress, and a key component of the Crusade was the objective of giving aid to fellow Christians in the East. Supporting this was the notion that the crusades were a justifiable holy work and therefore pleasing to God. The success of the First Crusade only added to the belief that these wars were “God-willed” (i.e., Deus vult, “God wills it”).

Prior to the Just War theory, Christians were largely pacifists. Doctrine of the Just War determined what violence was morally permissible. The Crusades, at the time, were understood as meeting the criteria of Augustine's Just War clause and were thought of as an act of love in response to Muslim aggression. In summary, violence needed to be in legitimate self-defence and adhere to principles concerning the following:

  1. Last resort

  2. Legitimate authority

  3. Just cause

  4. Probability of success (in order for a war to be just, there must be a rational possibility of success)

  5. Right intention (the primary objective of a just war is to re-establish peace)

  6. Proportionality (the violence in a just war must be proportional to the casualties suffered – only use the amount of force absolutely necessary)

  7. Civilian casualties (the use of force must distinguish between the militia and civilians – innocent citizens must never be the target of war)

We should also note that there was little racial or religious hatred against the Muslims. Although a lot of harsh propaganda and exaggeration was used, Muslims were largely unknown to the common people, so attempts to demonise them had a very limited effect. Crusaders actually felt conflicted after growing familiar with their opponents’ methods. Muslims were the enemy because they had made it difficult for Christians to visit holy sites, and they threatened Byzantium, not because they were Muslims. If it were the fact that they were Muslims, then the Crusades would’ve started much earlier. It is disingenuous to simply think of the crusades as a fanatical contest of faiths.

Alternative Motives

Beyond religious motives, there were a number of other reasons for people to join the crusades.

The Crusades, 1095-1204

For knight crusaders, there was the chance to win wealth, lands, and perhaps even a title. Some crusaders would stay in the reclaimed land and become lords of so-called “crusader states.” Note that this is an instance where things didn’t go as planned or intended. As I explained, land was supposed to return to Byzantium, but this was not always the case. While churchmen frowned upon worldly motives because they believed that such sinful aims would incur God's displeasure, many laymen had little difficulty in accommodating these alongside their religiosity.

Many knights were obliged to join their baron or lord as part of the service they performed to earn a living. Likewise, many knights followed their fathers or brothers, as ties of kinship and mutual protection were strong. There was, too, the idea of chivalry – that a knight should 'do the right thing' and protect not only the interests of their church and God, but also those of the weak and oppressed. Note, however, that during the First Crusade, this code was still in its infancy and so was more concerned with upholding a brotherhood of arms – its moral aspects would have a greater role with later crusades.

In a world in which firstborns inherited everything, it's entirely possible that second and third borns had something to prove and wanted to enhance their reputation or find a kind of inheritance equivalent elsewhere – there was wealth in crusading. Many merchants wanted to open up trade routes with the east – to enter into flourishing trade centres such as Antioch and Jerusalem. Merchants could profit from ferrying crusaders across the Mediterranean. For some peasants who chose to go on crusades, they might’ve seen a delay in their feudal service, a court case might’ve been sped up before departure, they might’ve been exempted from certain taxes, and the repayment of their debts could’ve been postponed. For some, they simply wanted revenge against Islam for being a consistent threat to the Christian Europe. As mentioned earlier, they basically started putting pressure on the Christendom from the beginnings of their faith. Others just had a general desire for honour and/or adventure.

We should remember that, while these may have been the motives of some individuals, there were many more who joined the crusades purely out of genuine religious zeal. In a time of such fervent religiosity, the city of Jerusalem held a central role. People were sensitive to communal pressure just as much as they feared for their own spiritual welfare. While medieval in nature, the crusades were not simply a baseless extremist religious war, but closer to a radical response to a cry for help. There were many who left their families and risked, and often lost, their lives in order to help their Orthodox brothers and reclaim the land on which Jesus lived.


[-] One might consider that "Killing in the name of God," when properly understood, is really the only kind of killing that could be legitimate. If God cannot legitimise killing, how could a state or any other creature? The problem is not that people take it too seriously, but that people don't take it seriously enough.

[-] Check out my online lesson plans for the crusades: If you're a high school humanities teacher, it may give you some helpful ideas (note that links may have expired on the site). If you're not a humanities teacher, feel free to check it out anyway! The information may interest you regardless.


  1. Bull, Marcus. “Origins.” In Oxford Illustrated History of the Crusades, edited by John Riley Smith, 13-33. Oxford: Oxford University press, 1995.

  2. Philip M. Taylor. “The Crusades.” In Munitions of the Mind, 73-. Manchester University Press, 2013.

  3. Baldwin, M. W. , Madden, . Thomas F. and Dickson, . Gary. “Crusades.” Encyclopedia Britannica, December 29, 2020.

  4. Cartwright, Mark. “The Crusades: Causes & Goals.” World History Encyclopedia, July 04, 2018.

  5. Portnykh, Valentin. “God Wills It! Supplementary Divine Purposes for the Crusades According to Crusade Propagranda.” The Journal of Ecclesiastical History 70, no. 3 (2019): 472-86.


Originally published at The Walk.



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