In 1095, Pope Urban II called upon European Christians to take up arms and reclaim Jerusalem from Muslim rule. Those who answered his call were promised spiritual rewards, including the remission of sins. The resulting military campaign became known as the First Crusade.
More than 900 years later, the current pope is delivering a very different message.
Speaking to an international gathering of religious leaders in Assisi, Italy, on October 27, Pope Leo XIV issued a sweeping condemnation of warfare conducted in God's name.
"There is no such thing as a 'holy war': only peace is holy," he declared.
The statement comes amid ongoing conflicts in Ukraine and the Middle East, where religious leaders and political figures have invoked divine authority to justify military action.
But Leo's declaration presents an uncomfortable historical question. If there are no holy wars, what are we to make of the countless wars that religious authorities have spent centuries declaring sacred?
When Killing Became a Christian Duty
According to the medieval chronicler Fulcher of Chartres, Urban II assured prospective crusaders that those who died fighting would receive immediate remission of their sins. His account also has the pope declaring, "Christ commands it."
The consequences were devastating. In 1096, crusading bands massacred Jewish communities along the Rhine. Three years later, Christian armies captured Jerusalem and slaughtered many of its Muslim and Jewish inhabitants.
Raymond of Aguilers, a Christian priest who accompanied the First Crusade, described the city littered with bodies. He celebrated the bloodshed as a "just and splendid judgment of God."
Writing in support of the Knights Templar, an order of Christian warrior-monks established during the Crusades, the influential theologian Bernard of Clairvaux declared:
"The Christian glories in the death of the pagan, because Christ is glorified."
The Crusades eventually expanded beyond battles against Muslim rulers. In 1209, Pope Innocent III launched the Albigensian Crusade against the Cathars, a fringe Christian religious movement in southern France that the Catholic Church considered heretical. The campaign resulted in widespread massacres and the destruction of entire communities.
Historical estimates suggest that between one million and nine million people died during the Crusades in the Holy Land between 1095 and 1291. Medieval historian Rebecca Rist places the likely toll closer to five or six million, including Christian and Muslim soldiers and civilians killed during nearly two centuries of warfare.
(Un)friendly Fire
Consider Joan of Arc, the 16-year-old French peasant who claimed that heavenly voices had instructed her to help restore the French king to his throne during the Hundred Years' War against England.
"I have done nothing save by revelation," she told an ecclesiastical court in 1431.
Joan helped inspire French military victories, but the English-backed court condemned her for heresy and burned her at the stake. Nearly five centuries later, in 1920, the Catholic Church canonized her as a saint.
Internal religious divisions became considerably more destructive during the Protestant Reformation.
In August 1572, Catholic forces and mobs massacred French Protestants during the St. Bartholomew's Day Massacre. Historian Mack P. Holt estimates that approximately 4,000 Protestants were killed in Paris, with another 3,000 murdered elsewhere in France. When news reached Rome, Pope Gregory XIII ordered a celebratory thanksgiving service. He also commissioned a commemorative medal depicting the destruction of the Protestant Huguenots.
The Thirty Years' War, fought from 1618 to 1648, was even more destructive. Beginning amid disputes between Catholic and Protestant powers, the conflict expanded into a struggle involving many of Europe's major states. An estimated five to eight million people died, most of them civilians, with starvation and disease accounting for much of the devastation.
Christian religious conviction also influenced England's military campaigns in Ireland.
In 1649, following the massacre of Royalist defenders at Drogheda, English military commander Oliver Cromwell wrote to Parliament describing the bloodshed as "a righteous judgment of God." His Protestant religious convictions helped shape his interpretation of England's military campaign in Ireland.
As we've explored previously, Christianity's history contains numerous examples of violence justified through religious belief, even as other Christians have drawn upon the same tradition to advocate peace.
Holy Wars Beyond Christianity
The Crusades may be the most famous examples of holy war in recorded history, but religiously sanctioned warfare has appeared across numerous traditions.
In Islam, the term jihad means struggle or striving. It encompasses personal spiritual and moral efforts, but historical Islamic traditions have also recognized armed jihad under certain circumstances.
In fact, during the medieval Crusades, Muslim leaders invoked jihad to rally resistance against Christian armies. Saladin, who recaptured Jerusalem in 1187, was among the Islamic rulers who appealed to religious devotion while pursuing military objectives.
In nineteenth-century West Africa, the Islamic scholar Usman dan Fodio launched his own jihad against the rulers of Gobir, in what is now northern Nigeria. He accused Muslim authorities of religious corruption and failing to govern according to Islamic principles. In a document explaining the obligations of Muslim believers, Usman declared that "jihad is a duty unanimously."
His movement established the Sokoto Caliphate, which became one of Africa's largest countries. This was a religiously justified conflict fought substantially among Muslims.
Another major African religious movement emerged in Sudan in 1881, when Muhammad Ahmad proclaimed himself the Mahdi, a messianic figure expected by many Muslims to restore justice before the end times.
Ahmad claimed divine authorization to overthrow Egyptian rule, reportedly declaring that his enemies could be purified only through force. His followers eventually conquered Khartoum and established a religious state that survived until British and Egyptian forces reconquered Sudan.
At the Battle of Omdurman in September 1898, approximately 10,000 Mahdist fighters were killed, compared with fewer than fifty Anglo-Egyptian soldiers.
The Buddhist Promise of Paradise
Religious warfare has also appeared in traditions frequently associated with nonviolence.
In sixteenth-century Japan, followers of Jōdo Shinshū Buddhism organized armed resistance against powerful regional warlords. The religious leaders of what would come to be known as the Honganji Movement encouraged their followers to fight with promises of spiritual rewards.
A banner associated with the movement proclaimed:
"Advance and be reborn in paradise; retreat and fall immediately into hell."
According to research published in the journal Public Choice, the promise of rebirth in the Buddhist Pure Land helped inspire loyalty among the movement's followers.
The Man Who Believed He Was Jesus' Brother
Perhaps the strangest and deadliest example of religiously inspired warfare occurred in nineteenth-century China.
Hong Xiuquan was a failed civil service examination candidate who claimed to have experienced a series of religious visions. After encountering Christian missionary literature, he became convinced that he was the younger brother of Jesus Christ, sent by God to cleanse China of evil.
Hong developed a religious movement combining elements of Christianity with his own revelations. In 1850, his followers launched the Taiping Rebellion against China's ruling Qing dynasty. In an 1853 proclamation, Taiping commanders accused their enemies of abandoning the true God, declaring:
"These demons are not at all human."
The Taiping armies captured major cities, including Nanjing, which became the capital of their self-proclaimed Heavenly Kingdom of Great Peace. The rebellion lasted until 1864, and by the end at least 20 million people had been killed. It's one of the deadliest conflicts in all of recorded history. Although mainstream Christian churches rejected Hong's teachings, the Taiping movement considered itself part of a divine mission. For a movement promising heavenly peace, the earthly consequences were catastrophic.
A History of Holy Wars
The following charts trace 30 historically significant religious conflicts across more than two thousand years, from wars explicitly declared sacred to campaigns in which faith motivated rebellion, persecution, or resistance.
Organized by religious tradition and historical period, they reveal how widely the phenomenon has appeared throughout recorded history.
What stands out is how frequently believers fought members of their own religion, rather than followers of competing faiths. Religious warfare was never exclusively a struggle between Christianity and Islam, nor did it disappear when the medieval era ended.
The historical record also invites an intriguing thought experiment.
What would our world map look like if wars fought in the name of religion had never occurred? Would Europe have the same borders? Would the partition of British India have unfolded differently? We can't reconstruct an alternate history, but the further down that rabbit hole you go, the more you realize how fundamentally different the globe could be.
Holy Wars Haven't Disappeared
Religious warfare may evoke images of medieval knights and ancient armies, but the modern world has hardly escaped its influence.
In 1998, Osama bin Laden and other Islamist militants issued a religious declaration calling for the killing of Americans, including civilians. They described this violence as "an individual duty for every Muslim who can do it," claiming religious authority to defend Muslim lands and holy sites.
Bin Laden was the founder of al-Qaeda, a Sunni extremist militant network that sought to overthrow governments it considered insufficiently Islamic and drive Western influence from Muslim-majority countries. Three years after that declaration, al-Qaeda terrorists killed 2,977 people in the September 11 attacks.
But prominent Muslim religious authorities rejected al-Qaeda's interpretation of Islam. Muhammad Sayyid Tantawi, the grand imam of Egypt's al-Azhar, one of Sunni Islam's most respected institutions, declared that bin Laden "doesn't represent Islam." Abdol Majid Khoei, a prominent Iraqi-born Shiite scholar, likewise condemned the attacks, insisting: "No Muslim should defend them."
Five days after the attacks, President George W. Bush used some strikingly familiar language to describe America's forthcoming response:
"This crusade, this war on terrorism is going to take a while."
The terminology prompted criticism given the historical association between the Crusades and Christian warfare against Muslims. The White House subsequently clarified that Bush had meant "crusade" in the general sense of a broad undertaking, not a religious war against Islam.
The United States invaded Iraq in 2003, primarily citing concerns about Saddam Hussein's alleged weapons of mass destruction. Iraq had played no role in 9/11, but the invasion gave Islamist militants an opening to a new battlefield. In 2004, a militant group led by Abu Musab al-Zarqawi pledged allegiance to bin Laden and became known as al-Qaeda in Iraq.
Zarqawi sought to provoke a religious civil war between Sunni and Shiite Muslims, deliberately targeting Shiite civilians and holy sites. In 2006, the bombing of a revered Shiite shrine in Samarra, widely attributed to al-Qaeda in Iraq, helped unleash widespread sectarian bloodshed.
Iraq's leading Shiite cleric, Grand Ayatollah Ali al-Sistani, called for restraint, urging followers not to retaliate against Sunni mosques. Across the wider conflict, Iraq Body Count has documented more than 200,000 civilian deaths from violence since the 2003 invasion.
More recently, in March 2024, the World Russian People's Council, chaired by Russian Orthodox Patriarch Kirill, approved a declaration characterizing Russia's invasion of Ukraine as a 'holy war.' The World Council of Churches strongly objected.
Religious rhetoric has also surrounded the continuing war involving Iran, where certain Christian interpretations of biblical prophecy and Islamic beliefs about the end times have entered public discussion.
Pope Leo's declaration therefore confronts a centuries-old argument that remains very much alive.
Who Gets to Declare a War Holy?
Catholic teaching still recognizes legitimate military defense under strict conditions, a concept known as just war theory. Leo's declaration draws a different line: a war might be morally defensible without being sacred or earning God's favor.
For centuries, religious leaders have offered their followers salvation, spiritual duty, or divine approval as reasons to fight. Other voices within those same traditions have said the opposite. The difference hasn't been merely theological. People have been asked to kill, and sometimes to die, on the strength of those claims.
If a religious leader declared tomorrow that God wanted you to go to war, would you take up arms?
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