A religious war is a war where the main cause is, or appears to be, religion or religious differences.The European Wars of Religion, the Crusades, and the Reconquista are frequently cited examples.

While there are undoubtedly wars fought primarily on religous grounds, wars frequently have multiple and complex causes. In modern times religious designations are frequently used as shorthand for cultural and historical differences between combatants, giving the impression that the conflict is primarily about religious differences. For example The Troubles in Northern Ireland are frequently seen as a conflict between Catholic and Protestant. However the more fundamental cause is the attachment of Northern Ireland to either the Republic of Ireland or the United Kingdom. As the native Irish were mostly Catholic, and the later English-sponsored immigrants mainly Protestant, the terms become shorthand for the two cultures. On the other hand it cannot be denied that religion does play a part in the conflict, as churches are used as organising points for demonstrations, and as Protestant church leaders vehemently oppose union with the Catholic-dominated Republic.

Jihad

The Islamic term jihad (literally "struggle") can refer to religious war ("lesser jihad"), though historically, the concept refers to an inward striving for perfect faith ("greater jihad").

There has come some dispute to the previous comment, whether the "greater" or "lesser" form of jihad refers to physical combat or an inner struggle. The debate comes from a hadith, or narration from the Prophet Muhammad, where he was quoted as saying, after returning from combat, "We have returned from the lesser jihad (jihad al-asghar) to the greater jihad (jihad al-akbar);" however, this report has received much criticism as being an inauthentic saying, i.e. it is a saying of someone after the death of Muhammad and was not the words of Muhammad himself.

Some view the greater jihad to be that of "the sword," or physical combat, using multiple references to the importance of fighting (al-qital) in the Qur'an and authenticated narrations from Muhammad as evidence. Beside all of this, both sides of the debate agree that the physical fight requires an initial overcoming of spiritual dismay, which is itself an inner struggle; thus, intellectually, one could derive that both struggles, the physical and the spiritual, are encompassing one of the other. Likewise, the spiritual success supposedly leads one to yearn to fight for the sake of God, whether in pure defense or preemptive strikes, so the actual fulfillment of such inner struggle is requisite to the purity of the outward struggle.

Programming usage

In computer circles the term religious war or holy war is used to describe a disagreement between the supporters of competing technologies or methodologies where the 'discussion' has gone beyond the level of comparison of the benefits of each to the point where fanatical supporters of each refuse to acknowledge weak points and insist that their side has the best solution in all cases.