A religious war is a war caused by, or justified by, religious differences. It can involve one state with an established religion against another state with a different religion or a different sect within the same religion, or a religiously motivated group attempting to spread its faith by violence, or to suppress another group because of its religious beliefs or practices. The Muslim conquests, the French Wars of Religion, the Crusades, and the Reconquista are frequently cited historical examples.
Many wars that are not considered religious wars can include elements of religion. Many "religious wars" had economic or political ramifications (land acquisition, control of trade routes, dynasty changes, etc.) that could call into question the true reasons behind the conflict. Differences in religion can further inflame a war being fought for other reasons. Historically, places of worship have been destroyed to weaken the morale of the opponent, even when the war itself is not being waged over religious ideals.
Religious designations are sometimes used as shorthand for cultural and historical differences between combatants, giving the often misleading impression that the conflict is primarily about religious differences. For example, there is a common perception of The Troubles in Northern Ireland as a religious conflict, as one side (Nationalists) was predominantly composed of Catholics and the other (Unionists) of Protestants. However, the more fundamental cause is the attachment of Northern Ireland to either the Republic of Ireland or the United Kingdom and while religion played a role as a cultural marker, the conflict was in fact 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 it is inaccurate to describe the conflict as a religious one.