[Sivran]

Palestinians and Israelis fight over "holy land" among other things.

Islamic fundamentlist extremists (note the extremist qualifier) fly planes into buildings, blow themselves up in malls, cars, and anywhere else they can cause a lot of pain.

And that's just off the top of my head without going very far back. You can't simply ignore the violence done by those claiming to be "doing god's work".

Religion, money/trade, resources, and survival. These are all primary causes for war. Any war can be put into one or more of these four boxes.