The people of premodern Christendom were no strangers to war. It featured prominently in their artwork, their literature, and their lore. The Song of Roland, for example, was a famous French epic poem that detailed a fictional, bloody conflict between the Christian Franks and Muslim Saracens in Spain. The early 11th century was punctuated by the Battle of Hastings, a day-long fray that ended with William, the Duke of Normandy, unseating Harold as the King of England. If one takes a cursory glance at medieval military history, all this fighting—whether deemed “holy” or “just”—seems to be the same. After all, most wars, whether real or not, were waged from a God-given moral high ground in the name of divinely sanctioned strife. But there is a key difference between Just War and Holy War: each has a distinct relationship to violence. In Just War, violence is an unfortunate, albeit necessary, means to an end. The fighters of a Holy War, on the other hand, actively pursue and laud violence—and even see it as an end unto itself.

Augustine of Hippo, a North African bishop and citizen of the Roman Empire, first introduces the concept of Just War in his fifth-century letters. In Letter 189, he asserts that “[p]eace should be the object of [one’s] desire; war should be waged only as a necessity.” 1 Later, he repeats this sentiment, explaining that soldiers should kill sparingly and only if there is no other alternative: “Let necessity, therefore, and not your will, slay the enemy who fights against you.” 2 In Just War, violence is simply a last resort—and an undesirable one, at that—to settle conflict and achieve peace.

In Letter 138, Saint Augustine even compares engaging in combat to disciplining a child. “For in the correction of a son...is received with reluctance and pain,” and in doing so “there is assuredly no diminution of a father’s love.” 3 Warfare is an act imbued with “reluctance and pain,” but will ultimately contribute to the greater good, as its core objective is a state of lawfulness and concord.

In Just War, violence should also be tempered with compassion. “As violence is used towards him who rebels and resists,” Augustine writes, “so mercy is due to the vanquished or the captive, especially in the case in which future troubling of the peace is not to be feared.” 4 Violence may be obligatory, Augustine seems to say, but clemency is also important virtues in war. Just War is only “just” because it is carried out in the name of peace—if peace has been realized, then any further violence is superfluous.

One can see Augustine of Hippo’s theory of Just War play out in William of Poitiers’s account of the Battle of Hastings. According to this Frankish priest, the former king of England, Edward the Confessor, had designated William of Normandy as his rightful heir. 5 This position, however, was usurped by Harold, whose transgression was twofold: he was both an illegitimate successor and a selfish, corrupt leader. Harold was “blinded by his lust for dominion” and “unmindful of the wrongs he had committed.” 6 William of Poitiers, therefore, repeatedly emphasizes to his readers that the Battle of Hastings was an unambiguously Just War, fought to secure William of Normandy’s rightful place on the throne. He writes that the Duke of Normandy had “full assurance that Almighty God would not allow wrong to prevail nor a just cause to fail” 7 and even explicitly refers to the conflict as a “just war.” 8 But the most defining feature of this Just War was William of Normandy’s understanding of violence.

In direct accordance with Augustine of Hippo’s doctrine, William of Normandy insists that if either the law of England or the law of Normandy dictates that “the kingdom is [Harold’s] by right,” then Harold can “possess it in peace.” 9 He states that violence is not his end goal, but rather acquiring England “in justice.” 10 The Duke of Normandy sagely explains that he does “not think it right that either [his] men or [Harold’s] should perish in a conflict over a quarrel that is none of their making.” 11 In fact, just as Saint Augustine exhorts, William of Normandy seeks to minimize the violence, declaring: “I am therefore ready to risk my life against his in a single combat to decide whether the kingdom of England should by right be his or mine.” 12 And so the Battle of Hastings commences, as an alternative to a bloodier, full-fledged war.

What’s more, when the battle has finally ended, William of Normandy does not rejoice in his victory and the death of his enemies. Instead, he solemnly looks out upon the battlefield and “[cannot] gaze without pity on the carnage, although the slain were evil men.” 13 Just as Augustine of Hippo, he views war as a necessary means by which to mete out justice. Even though the battle’s cause is noble and even compulsory, the actual loss of life evokes sorrow and sympathy. This excerpt is a clear example of the “mercy [that] is due to the vanquished or the captive” that Augustine of Hippo describes in his letters. William of Normandy did not want to pursue violence, but did so on behalf of a Christian notion of justice.

In a Holy War, on the other hand, violence is a virtuous, actively sought-after act. In chapter 20 of the book of Deuteronomy, which lays out the rules of warfare, God instructs His followers to “put to the sword all the men” in a city that refuses to convert to Christianity. 14 “Do not leave alive anything that breathes,” the scripture continues. 15 “Otherwise, they [the non-Christians] will teach you to follow all the detestable things they do in worshiping their gods, and you will sin against the Lord your God.” 16 Violence is not only acceptable in Holy War—it is righteous. To abstain from annihilatory violence would be to “sin against the Lord.” This sentiment is echoed in 1 Samuel 15, in which Samuel, relaying God’s message to Saul, instructs him to “attack the Amalekites and totally destroy all that belongs to them.” 17 His perspective on violence is made abundantly clear: “[d]o not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys.” 18 When Saul does not follow through on this commandment, he provokes God’s wrath.

A prime example of Holy War can be seen in 11th-century epic poem The Song of Roland, in which violence between the Christian Franks and the Muslims Saracens is deemed honorable and praiseworthy. The author spends the better part of the book detailing the bloody battle, painting the Franks—and at times, even the Saracens—as lionhearted heroes.

But the Muslim adversaries are also depicted as inhuman savages. The Milceni, a group within the Muslim army, “sprout bristles just as pigs do” on their spines. 19 The men from Occian “refus[e] to serve” the Christian God, have “hides [that] are rock hard, just like iron,” and “[i]n battle they are treacherous and bloodthirsty.” 20 And when the Saracen King Marsilie dies, he “gives up his soul to the devils incarnate.” 21 Killing the Muslims, therefore, is a hallowed act. After all, they are depicted as more demon than human in the first place, as indicated by their bristling spines and iron hides. According to the Christian Frank perspective, since these religious and racial “others” spurn God’s name and hail from hell, murdering them is a righteous act and a fulfillment of the Lord’s will. In fact, the author of The Song of Roland explicitly writes that “God does not wish [the pagans] to remain.” 22 Each death of a pagan Muslim brings honor, both on earth and in heaven, to the Frank army. One could even argue that the object of a Holy War violence itself—a significant difference from Just War, which fundamentally aims to bring about peace.

Compared to Saint Augustine’s insistence that “war is waged in order that peace may be obtained,” the battle in The Song of Roland overtly glorifies bloody violence. Of course, this difference may be partly attributed to the fact that the English, unlike the Muslim Saracens, were not racial or religious “others,” and were seen in their full humanity as equal counterparts to the French. But the fact that William of Normandy laments the loss of English life while King Charlemagne actively endorses the deaths of thousands of his adversaries is significant.

Although both Holy and Just Wars are fought in the name of Christian morals, both exhibit notably different outlooks on violence. In Just War, violence is an undesirable means to a desirable end of peace and tranquility. In Holy War, however, violence is a sacrosanct and virtuous act. And if your enemies are “treacherous” Muslims, this violence may as well be the end in and of itself.