(This is a lightly amended version of another contribution I made in an online forum elsewhere; nobody else seemed to pick up on this idea – maybe it was just too serious for the forum in question – so I’m floating it again via this blog)
The point almost everyone in the original forum seemed to be missing is that to have a war involving religion, the religion in question must be involved in a state or nation – or trying forcibly to be so. Also the religion must accept warfare as a valid ethical option. Given that presupposition, yes there are times when religion causes war, times when it is used as a pretext, and times when different religions on each side of a war means that a dispute which is perhaps really about something else becomes extra-intransigent because neither side can surrender a cause they have come to see as their god’s cause – as witness the cry ‘No Surrender’ in Ulster.
Most religions have started as national and so are integral with initially a particular ethnic group and later a large territorial state, and so naturally become involved in their state’s wars (including, as in 17th Century England, civil wars). Of what might be called the classic religions, only two are not thus limited, at least in their origins – Buddhism and Christianity. Buddhism has become a national religion in some states (e.g., Tibet) and so involved in wars, but really should be pacifist because of its philosophical view of the world.
Christianity should be pacifist because Jesus said at square one his ‘kingdom’ is not ‘of this world’, and the subjects of the Kingdom are those ‘born again’ through faith of any race or nation. The New Testament depicts Christians living as peaceable ‘resident aliens’ in whatever nation, their native land or another, a people who may be engaged in a spiritual war but their warfare is ‘not with physical weapons’. So long as Christians stick to that New Testament view of their faith the Christian religion cannot cause wars – though they may be persecuted for their non-conformity in many states.
Only some 300 years after Jesus did Christianity get ‘nationalised’ by a Roman Empire trying to replace a pagan religion which had lost credibility. Only then could Christianity become a war-involved religion – but of course in serious disobedience to the Christian God.