Aliens... They get tired of watching from above and either svck all the oxygen from the atmosphere and fly away, or pull a 2001:Space Odyssey and come down to teach humanity the tools and tricks to finally overcome it's never-ending attraction to war.
The root causes of war are basically quality of life, and scarcity of resources (which is the cause of low quality of life).
War will always exist until resources are no longer a problem, something that is completely improbable.
If you get down to the root sources of terrorism in the world right now, it basically comes down to low access to females (cause by large wealth disparities and polygamy). All of the rich guys get multiple wives, and the poor guys sometimes never get married. The rest of their life is [censored], so they have nothing to live for, they are the "have-nots". They see the rich countries on the TV getting all the things they want in life only irritates it, so the enemy becomes the "haves". We are not the cause, but it's usually the straw that broke the camel's back.
So basically, war will always exist, at least as long as their are beings intelligent enough to wage it. Simply put, it's an evolutionary trait, beneficial for survival in the wild.
Hell, all of the modern research suggests we are born with these tendencies, and that we are in fact BORN RACIST.