[Acheron]

I'm not sure that it's possible to disentangle the causes of a war. While proximate causes are fairly easy to work out (for example, the Second World War began because Germany invaded Poland), ultimate causes are trickier. Religion, if it's a cause of war at all, can only be an ultimate cause: religion is an ongoing idea rather than a single event. And while I think that in many cases religion has been part of the motivation for war, it goes hand in hand with political and economic factors. Could those wars in which religion was a factor have taken place without religion? Impossible to say. Religion has been such a constant factor in human civilisation that it's removal would lead to societies very different to the ones that have evolved in our religion-infested world.

Still, I suspect it is a major cause for conflict. An essential part of warfare, and other forms of conflict, is to see your enemy not as someone largely similar to yourself but as someone alien. That's much easier if you see them as a hell-bound sinner, hated by your god for all the abominable acts they commit.