A religious war or holy war (Latin: bellum sacrum) is a war primarily caused or justified by differences in religion. In the modern period, debates are common over the extent to which religious, economic, or ethnic aspects of a conflict predominate in a given war. According to the Encyclopedia of Wars, out of all 1,763 known/recorded historical conflicts, 123, or 6.98%, had religion as their primary cause. Matthew White's The Great Big Book of Horrible Things gives religion as the cause of 13 of the world's 100 deadliest atrocities. In several conflicts including the Israeli–Palestinian conflict, the Syrian civil war, and the wars in Afghanistan and Iraq, religious elements are overtly present but variously described as fundamentalism or religious extremism—depending upon the observer's sympathies. However, studies on these cases often conclude that ethnic animosities drive much of the conflicts.
Some historians argue that what is termed "religious wars" is a largely "Western dichotomy" and a modern invention from the past few centuries, arguing that all wars that are classed as "religious" have secular (economic or political) ramifications. Similar opinions were expressed as early as the 1760s, during the Seven Years' War, widely recognized to be "religious" in motivation, noting that the warring factions were not necessarily split along confessional lines as much as along secular interests.
According to Jeffrey Burton Russell, numerous cases of supposed acts of religious wars such as the Thirty Years' War, the French Wars of Religion, the Sri Lankan Civil War, 9/11 and other terrorist attacks, the Bosnian War, and the Rwandan Civil War were all primarily motivated by social, political, and economic issues rather than religion.
The war of Egyptians with Israelis, Pakistanis versus Hindus, and the conflict between Turkish-origin and Greek-origin Cypriots are recent instances where the ideals of Jihad have been introduced.
The tendency for a nation to arrange divine endorsement of its defensive or offensive wars is known from antiquity, and was probably a characteristic of primitive man as exemplified by the American Indian war dances and other rites invoking supernatural help. However there is a difference of scale when one moves from the single, tribal level to levels of tribal confederations or nations. Religious fervor invoked on this scale can abet an enormous amount of bloodletting: the Crusades illustrate this. Islamic emphasis on the sword is seen by the conquests in its first thousand years. The wars of the Turkish Ottoman Empire (1300-1922), while not specifically Jihads, show such characteristic features as the desecration of the Hagia Sophia after the capture of Constantinople (1453); forced conversions to Islam over the centuries; and the genocidal massacres of over 2 million Catholic Armenians (1894-1896, 1915-16). Even where Jihad is not officially proclaimed, fighters may call themselves 'mujahidin' or holy warriors and circumvent the Koranic protection extended to Christians and Jews, as in the Six Days War with Israel.
Pope Urban, in calling for the first crusade, did not know how the crusaders would behave, and thought he was defending Christians in the East.