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For millennia, religion has been one of the many reasons for war. The use of force to impose a religious belief might sound fairly barbaric, but humankind has been practicing this since the dawn of time. From Christian denominations fighting amongst themselves to the persecution of Muslims, Christians, and Jews, amongst others, history is tainted with bloody religious conflicts that claimed millions of lives.

Browse through this gallery and learn more about the deadliest religious conflicts in history.

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The Reconquista was all about Christians conquering territory from Muslims in the Iberian Peninsula (Spain and Portugal). Its beginnings were marked by the victory of Don Pelayo of Asturias at Covadonga around 720 CE.

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This series of wars lasted for 780 years, and it's estimated that around seven million people died as a result.

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The First Crusade was declared in 1095. The idea was to seize Jerusalem from Islamic rule. Though this was just the beginning of the Roman Catholic Church's campaign, and it eventually turned into a 300-year-long bloodbath.

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More Crusades followed, and while it's difficult to calculate the number of deaths during the First Crusade, in total it is estimated that about 1.7 million lives were lost during the whole period of the Crusades.

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While mostly Muslims and Christians died during this period, Jewish people also fell victims of the Crusaders.

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The French Wars of Religion took place in the 16th century. The country was pretty much divided between Catholics and Huguenots (Protestants).

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Perhaps the most infamous event of the time was the St. Bartholomew's Day Massacre of 1572, where a mob of Catholics attacked Huguenots.

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The wars ended when Huguenot Henry III of Navarre became King Henry IV of France and converted to Catholicism.

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In 1598, the 'Edict of Nantes' was issued, granting religious freedom and rights to Huguenots, marking the end of the 36-year-long conflict.

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This European war took place in the Holy Roman Empire and lasted from 1618 to 1648. It started as a Catholics versus Protestants conflict in Prague, and it escalated from there.

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It's regarded as one of the most devastating wars in European history, with an estimated eight million lives lost. But there were more causalities than just battlefield deaths.

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The war happened during a cold snap in Europe, meaning that low temperatures affected agriculture, leading to food shortages and consequently famine and more deaths.

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This conflict took place when Orthodox Russia expanded towards the Black Sea in the 18th century, clashing with the Islamic Ottoman Empire.

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The Russians faced resistance from the peoples of the North Caucasus. The Muslim Caucasian mountaineers fought off the Orthodox Russians between 1763 and 1864.

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When the Russians' military strategy failed, they evicted the Caucasians from their homelands. In 1864, almost three million Muslim Adyghe, Ubykhs, and Abkhaz were sent to the Ottoman Empire on ships. Many of those sank.

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There was a strong anti-Catholic sentiment during the French Revolution. The whole dechristianization of the country was among the goals of the revolutionaries.

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One of the laws imposed by the revolutionaries was the Civil Constitution of the Clergy, where all Catholic clergy had to swear loyalty to the state.

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But not every French person was in favor of the revolution, and many fought back. It is estimated that French forces massacred over 50,000 people during the Reign of Terror alone.

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This war against the Manchu imperial dynasty began in 1850 and lasted 14 years. It was instigated by a Chinese revolutionary named Hong Xiuquan.

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While the dispute was mostly territorial, Hong Xiuquan claimed to be Jesus Christ's younger brother, whose mission it was to fight the Confucian demons, i.e. the Manchu empire.

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Xiuquan established the Heavenly Kingdom of Great Harmony (Taiping Tianguo), but his rebellion was eventually defeated. It is estimated that around 20 million people died during the conflict.

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The Bolshevik Revolution was instigated by socialists (later communists) against tsarists. The socialists/communists were militant atheists, and as such sought to eliminate all religion, most notably the Russian Orthodox Church.

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Though religion was never banned, the USSR later targeted religious communities who were connected to the tsarist regime. It is estimated that around 30 million died in the Bolshevik Great Terror of 1936–1938.

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The Armenian Genocide by the Ottoman Empire took place in 1915 and resulted in the slaughter of thousands of Armenian people and their identity.

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But the Armenian Genocide was just one event in the Ottoman Empire's persecution of Christian communities between 1914 and 1923. It started in 1914 with the massacre of Assyrian Christian communities, followed by the Armenians. By 1918, over 600,000 Assyrians and 1.5 million Armenians had died.

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The partition of India took place in 1947 and divided British India into the modern states of India and Pakistan.

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Violence between Hindu, Muslim, and Sikh groups followed. The conflict was paired with mass migration.

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It's estimated that around two million people were killed, and at least 15 million were displaced.

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South Sudan was born in 2011, out of a bloody conflict between the Christian and animist south and the Muslim north.

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It is estimated that at least two million lives were lost to the conflict, and hundreds of thousands of people were displaced.

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The Syrian Civil War began in 2011, following the Arab Spring of 2010. Syrian President Bashar Assad's opposition was composed of (among others) radical Islamists, including ISIS.

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The war spread to Iraq, where ISIS targeted minority groups, namely Yazidis and Assyrian Christians. Shia Muslim Turkmen were also killed. It is estimated that around 5,000 Yazidis were killed, 400,000 were displaced, and 6,000 were enslaved.

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While the Syrian Civil War didn't begin as a religious conflict, it evolved into one as religious beliefs became tied to the various militant groups fighting to overthrow Assad. Finally, in December 2024, the Islamist militant group Hayat Tahrir al-Sham took control of several major cities in Syria before entering the capital of Damascus. Bashar al-Assad fled to Russia in fear, effectively giving up leadership of the country. The Syrian people are celebrating the end of decades of oppression under the Assad family regime, but the future of the country remains uncertain.

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While the conflict between Israel and Palestine is about much more than religion, religion still plays a major role in the war that is currently tearing the Middle East apart. The history of Israel's occupation of Palestine goes back to 1948, but the situation escalated dramatically on October 7, 2023, when Hamas launched an attack on Israeli soil that killed 1,400 people. Hamas is another name for the Islamic Resistance Movement, an Islamist political and military organization that has been ruling over the Gaza Strip since 2007. Hamas has been condemned as a terrorist organization by much of the Western world, but they claim that they are simply fighting violence with violence to defend Palestine from Israel's ongoing advances.

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Israel responded to the October 7 attack with a relentless air bombardment and ground invasion that has carried on for over a year, killing tens of thousands of Palestinians, many of them children. As the war rages on, public discourse surrounding the conflict becomes muddled with Islamophobic and anti-Semitic rhetoric. Condemning or supporting either side is being conflated with religious discrimination, when in reality, most are just calling for an end to the violence and senseless loss of life. At the same time, those who already harbored anti-Semitic or Islamophobic views are capitalizing on this opportunity to spread hate and further confuse a debate that is already extremely emotionally charged.

Sources: (Grunge)