I don't actually think religion is the cause of war. It is used as a weapon or a pretext. But as far as cause goes, that's just human plain and simple. You can insert Nationalism, Patriotism and racism in that slot as well. They are tools.
What is it that Napoleon said. "A man does not willingly die in battle for a petty distinction or a medal, you must speak to the soul to electrify him" He knew what he was talking about. He replaced religion with Nationalism and it worked a charm. Again just a tool to rally the troops.
Many many Muslims would agree that those who are declaring jihad all over the place are completely in the wrong and not religious at all. If they were religious they would stay home and pray. What the middle east is caught in today is the ancient system of vendetta and nomadic warfare. Been going on for millennium. Religion is just an overlay and then ultimately used as a pretext.
"Hey look at what that guy has got....lets go get that"
I think the commonality in all wars is...............If somebody wants war, they'll go to war for any reason and make it sound good. Or make it sound good enough for the masses to join in.
There is a really good book called "War of the World" by Nials Ferguson. I like the book, but I think the author is kind of a twat. The Brits may know where I'm coming from. He claims all war is racist. But again, he's not conclusive either.
Wagon