Religion is not a cause. Nor greed, peace, blame, resources, land, or wealth. It's something far beyond that. Pride.
A lot of people can't handle being wrong about something so dear and close to them, so they simply attack people with different opinions in the name of their religion, land, or whatever they use to justify themselves.
Too prideful to accept that other people are different. Too prideful to accept that you may never be a name known to all. Too prideful to accept that you're poor. Too prideful to change your ways. Too prideful to accept that the world may never change.
Acceptance is the first step to unification.
It's kind of funny really, the more people try for something, the more it slips away. It's not impossible though. Everyone just needs to be united under a goal, everyone needs to work together. Arguing over heads or tails will never change anything, as it's simply a flip of the coin.
Then again, there is the idea that ambitious people try so hard for their dreams, that they may push other people away. Of course, everyone deserves the chance to work hard for something, and unique is a good thing. But if everyone was under one goal, what then, would the point be in individuality?
Does individuality cause war? The fear of uniformity? If everyone did one thing, all individuality is gone. No more diversity. Is individuality something we must sacrifice for total peace? I sure hope not. I'm sure there is a way for the world to unite and keep the people unique. The world has several goals it needs accomplishing (Energy Crisis, Space, Water Crisis, Pollution, etc.)
Currently, it's goals are all country-based, but if everyone broadened there horizon to what can really cause trouble, then work could be done to overcome it. As long as people are united under goals, their will be peace. When a goal is finally reached, simply try for another one.
Just keep selflessly working for a brighter future for others to have.
I dunno, it's all so complicated and far above me, I couldn't begin to comprehend it. :/