Religious war
Definitions: Konrad Repgen (1987) pointed out that belligerents may have multiple intentions to wage a war, may have had ulterior motives that historians can no longer discover, and therefore, calling something a 'religious war' (or 'war of succession') based merely on a motive that a belligerent may have had, does not necessarily make it one. Although ulterior motives may never be known, war proclamations do provide evidence for a belligerent's legitimisation of the war to the public. Repgen therefore concluded: 

...wars should only be termed [religious wars], in so far as at least one of the belligerents lays claim to 'religion', a religious law, in order to justify his warfare and to substantiate publicly why his use of military force against a political authority should be a bellum iustum.
Philip Benedict (2006) argued that Repgen's definition of 'religious war' was too narrow, because sometimes both legitimisation and motivation can be established. David Onnekink (2013) added that a 'religious war' is not necessarily the same as a 'holy war' (bellum sacrum): "After all, it is perfectly acceptable to suggest that a worldly prince, say, a Lutheran prince in Reformation Germany, engages in religious warfare using mercenary armies." While a holy war needs to be authorised by a religious leader and fought by pious soldiers, a religious war does not, he reasoned. His definition of 'war of religion' thus became:

a war legitimised by religion and/or for religious ends (but possibly fought by secular leaders and soldiers).

Applicability of religion to war: Some commentators have questioned the applicability of religion to war, in part because the word "religion" itself is difficult to define, particularly posing challenges when one tries to apply it to non-Western cultures. Secondly, it has been argued that religion is difficult to isolate as a factor, and is often just one of many factors driving a war. For example, many armed conflicts may be simultaneously wars of succession as well as wars of religion when two rival claimants to a throne also represent opposing religions. Examples include the War of the Three Henrys and the Succession of Henry IV of France during the French Wars of Religion, the Hessian War and the War of the Jülich Succession during the Reformation in Germany, and the Jacobite risings (including the Williamite–Jacobite wars) during the Reformation in Great Britain and Ireland.
John Morreall and Tamara Sonn (2013) have argued that since there is no consensus on definitions of "religion" among scholars and no way to isolate "religion" from the rest of the more likely motivational dimensions (social, political, and economic); it is incorrect to label any violent event as "religious".
Theologian William T. Cavanaugh in his Myth of Religious Violence (2009) argues that the very concept of "religion" is a modern Western concept that was invented recently in history. As such, he argues that the concept of "religious violence" or "religious wars" are incorrectly used to anachronistically label people and conflicts as participants in religious ideologies that never existed in the first place. The concept of "religion" as an abstraction which entails distinct sets of beliefs or doctrines is a recently invented concept in the English language since such usage began with texts from the 17th century due to the splitting of Christendom during the Protestant Reformation and more prevalent colonization or globalization in the age of exploration which involved contact with numerous foreign and indigenous cultures with non-European languages. It was in the 17th century that the concept of "religion" received its modern shape despite the fact that the Bible, the Quran, and other ancient sacred texts did not have a concept of religion in the original languages and neither did the people or the cultures in which these sacred texts were written. The modern word religion comes from the Latin word religio which, in the ancient and medieval world, was understood as an individual virtue of worship, never as doctrine, practice, or actual source of knowledge. Cavanaugh argued 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.
There is no precise equivalent of "religion" in Hebrew, and there is no clear definition of jewishness, it could be defined by religion, roots of national origin and ethnic. Jewishness could have been multi-racial. In the Quran, the Arabic word deen is often translated as "religion" in modern translations, but up to the mid-17th century, translators expressed deen as "law".
It was in the 19th century that the terms "Buddhism", "Hinduism", "Taoism", and "Confucianism" first emerged. Throughout its long history, Japan had no concept of "religion" since there was no corresponding Japanese word, nor anything close to its meaning, but when American warships appeared off the coast of Japan in 1853 and forced the Japanese government to sign treaties demanding, among other things, freedom of religion, the country had to contend with this Western idea. According to the philologist Max Müller, what is called ancient religion today, would have only been understood as "law" by the people in the ancient world. In Sanskrit word dharma, sometimes translated as "religion", also means law. Throughout the classical Indian subcontinent, the study of law consisted of concepts such as penance through piety and ceremonial as well as practical traditions. Medieval Japan at first had a similar union between "imperial law" and universal or "Buddha law", but these later became independent sources of power.
According to McGarry & O'Leary (1995), it is evident that religion as one aspect of a people's cultural heritage may serve as a cultural marker or ideological rationalization for a conflict that has deeper ethnic and cultural differences. They argued this specifically in the case of The Troubles in Northern Ireland, often portrayed as a religious conflict of a Catholic vs. a Protestant faction, while the more fundamental cause of the conflict was supposedly ethnic or nationalistic rather than religious in nature. Since the native Irish were mostly Catholic and the later British-sponsored immigrants were mainly Protestant, the terms become shorthand for the two cultures, but McGarry & O'Leary argued that it would be inaccurate to describe the conflict as a religious one.
In their 2015 review of violence and peacemaking in world religions, Irfan Omar and Michael Duffey stated: "This book does not ignore violence committed in the name of religion. Analyses of case studies of seeming religious violence often conclude that ethnic animosities strongly drive violence."

Prevalence: The definition of 'religious war' and the applicability of religion to war have a strong influence on how many wars may be properly labelled 'religious wars', and thus how prevalent religious wars have been as opposed to other wars.
According to Kalevi Holsti (1991, p. 308, Table 12.2), who catalogued and categorised wars from 1648 to 1989 into 24 categories of 'issues that generated wars', 'protect[ion of] religious confrères' (co-religionists) was (one of) the primary cause(s) of 14% of all wars during 1648–1714, 11% during 1715–1814, 10% during 1815–1914, and 0% during 1918–1941 and 1945–1989. Additionally, he found 'ethnic/religious unification/irredenta' to be (one of) the primary cause(s) of 0% of all wars during 1648–1714 and 1715–1814, 6% during 1815–1914, 17% during 1918–1941, and 12% during 1945–1989.
In their 1997 Encyclopedia of Wars, authors Charles Phillips and Alan Axelrod documented 1763 notable wars in world history, out of which 121 wars were in the "religious wars" category in the index. They note that before the 17th century, much of the "reasons" for conflicts were explained through the lens of religion and that after that time wars were explained through the lens of wars as a way to further sovereign interests. Some commentators have concluded that only 123 wars (7%) out of these 1763 wars were fundamentally originated by religious motivations. Andrew Holt (2018) traced the origin of the "only 123 religious wars" claim back to the 2008 book The Irrational Atheist of far-right activist Vox Day, which he notes is slightly adjusted compared to the 121 that is indeed found in the Encyclopedia of Wars itself.
The Encyclopedia of War, edited by Gordon Martel, using the criteria that the armed conflict must involve some overt religious action, concludes that 6% of the wars listed in their encyclopedia can be labelled religious wars.

Holy war concepts in religious traditions: While early empires could be described as henotheistic, i.e. dominated by a single god of the ruling elite (as Marduk in the Babylonian empire, Assur in the Assyrian empire, etc.