The Theological Foundations of Just War

Long before the Crusades reshaped medieval Europe, Christian thinkers grappled with the morality of warfare. In the 4th century, St. Augustine formulated the concept of bellum justum (just war), arguing that violence could be morally permissible under specific conditions. By the High Middle Ages, this theory had merged with Christian royal ideology, creating a framework where kings were expected to avoid conflict but could legitimately wage war to defend their realms or protect the vulnerable.

The Bible provided conflicting messages about warfare. While Christ famously told Peter to “put your sword back into its place” (Matthew 26:52), the Old Testament celebrated divinely sanctioned battles like Joshua’s conquest of Canaan. Medieval theologians reconciled these passages through the “Two Swords” doctrine: the Church wielded spiritual authority (excommunication, sacraments), while secular rulers held temporal power (military force). However, increasingly, clerics argued the Church should control both swords—determining when and how wars could be fought.

Chivalry and the Rules of War

By the 11th century, elite warriors followed an evolving code of chivalry that prohibited attacking non-combatants—especially women, children, clergy, and the elderly—unless they aided enemies. These ideals spread through movements like the Peace and Truce of God, which sought to limit feudal violence. Before battle, armies participated in elaborate religious rituals: fasting, prayers, blessings, and singing psalms like “Some trust in chariots and some in horses, but we trust in the name of the Lord our God” (Psalm 20:7). Victories were framed as divine judgments, with conquered lands often redistributed to victors, echoing Joshua’s conquests.

The Road to Clermont

Several factors converged to make holy war inevitable. Pilgrimages to Jerusalem surged in the 11th century, with some groups numbering thousands. Meanwhile, reports of Muslim atrocities—like the 1086 display of Christian heads after the Battle of Sagrajas—fueled outrage. Pope Gregory VII envisioned leading an army to rescue Eastern Christians, but the Investiture Controversy derailed his plans.

His successor, Urban II, faced a weakened papacy and saw opportunity in uniting Christendom against a common enemy. At the 1095 Council of Clermont, he delivered a rousing sermon (exact wording remains debated), urging knights to cease infighting and instead liberate Jerusalem. The crowd reportedly erupted with cries of “Deus vult!” (“God wills it!”). Urban promised spiritual rewards, likely including indulgence for sins, though whether he offered full remission remains disputed.

The People’s Crusade and Jewish Persecution

Enthusiasm outpaced organization. Months before nobles could mobilize, waves of peasants and minor knights—led by figures like Peter the Hermit—set off in spring 1096. Lacking supplies and discipline, these “People’s Crusades” often devolved into violence. In the Rhineland, Count Emicho’s followers massacred Jewish communities, forcing conversions or facing mass suicide. At Mainz, Jews killed their own families rather than submit, an act later memorialized in liturgical poetry. These atrocities strained Christian-Jewish relations for centuries and inspired Jewish pietist movements like the Hasidei Ashkenaz.

The Princes’ Crusade and the Siege of Antioch

The better-prepared noble armies departed in late 1096. Key leaders included Godfrey of Bouillon, Raymond of Toulouse, and Bohemond of Taranto. After swearing to return Byzantine lands, they crossed into Anatolia in 1097. Despite infighting, they won victories at Nicaea and Dorylaeum. Baldwin of Boulogne carved out the County of Edessa—the first Crusader state—while the main force besieged Antioch for eight grueling months.

A discovered “Holy Lance” (purportedly Christ’s crucifixion weapon) boosted morale, and the Crusaders broke the siege in June 1098. Byzantine allies, misjudging the situation, withdrew, deepening tensions between East and West.

The Sack of Jerusalem

By June 1099, only 15,000 exhausted Crusaders reached Jerusalem. On July 15, they stormed the city, committing horrific massacres. Chronicler Fulcher of Chartres wrote:

“In the Temple nearly 10,000 were beheaded. They spared no one, not even women or children.”

Godfrey became “Defender of the Holy Sepulchre,” and a surprise victory at Ascalon secured the kingdom. The Crusader States—Jerusalem, Antioch, Edessa, and later Tripoli—emerged as hybrid societies, blending European feudalism with local customs.

The Military Orders and Lasting Legacy

To protect pilgrims, new military orders arose: the Knights Hospitaller (founded c. 1099) and Knights Templar (1119). These monk-warriors became powerful international organizations, praised by figures like St. Bernard of Clairvaux.

The First Crusade’s success revitalized the medieval Church and created enduring legends. Returning nobles embellished their roles, while families invented crusading ancestors to burnish prestige. For Jews, 1096’s trauma inspired religious renewal and stricter separation from Christian society.

Ultimately, the Crusades reshaped Europe’s relationship with warfare, faith, and the wider world—a legacy still felt today in conflicts framed as holy causes. The line between just war and holy war, first blurred at Clermont, remains contested.