Lets get down to brass tacks here, religion is NOT the cause of war, it's the rationale for war. The sad fact is that all war (in fact all human conflict really) is fought for the same reason; securing resources. One tribe fights another to secure hunting territory, the Norsemen pillage and loot villages, the Crusaders plunder the riches of the Moors, the south succeeds from the union to keep their slaves and economy, and Cheney wants his oil from Iraq.
But you can't get enough people to put themselves in front of a bullet just for resources so they need a greater cause. That's where religion comes in, that's where pride comes in, that's where nationalism, patriotism, and a sense of duty (however well founded or not) comes in to make war seem necessary.
People's need to believe is simply a mechanism of war, not the reason for it.
Law 27: Play on People's Need to Believe to Create a Cultlike Following
People have an overwhelming desire to believe in something. Become the focal point of such desire by offering them a cause, a new faith to follow. Keep your words vague but full of promise; emphasize enthusiasm over rationality and clear thinking. Give your new disciples rituals to perform, ask them to make sacrifices on your behalf. In the absence of organized religion and grand causes, your new belief system will bring you untold power.