World government run by a dictator with an iron fist and full support of the military/security forces would go a long way toward ending war
But good luck making that happen, and if you did, a lot of people would miss the good old days of war anyway
It's pretty hard to pin down the causes of war, or actually even define a state of war. Realists believe that war is a product of the international system i.e. anarchy and the balance of power between sovereign nation states. In fact, most conventional international relations theory assumes that a state of peace exists when the great powers aren't fighting each other (the millions dying elsewhere don't really count and are peripheral to relevant international politics). As such, the period from the end of WWII until now is generally considered to be relatively peaceful.
Liberalism suggests that peace will spread as liberal democracy spreads because of shared ideals and economic interests, and they'll back this up by suggesting that no liberal democracy has ever gone to war against another liberal democracy. History does back this up, but it's a very short history, and doesn't take into account that there was often a major shared threat i.e. nazism or communism that might keep the liberal democracies away from each other's throats. It also doesn't seem to take into account that Liberal democracy is probably as much a cultural thing as anything else and won't be accepted by everyone willingly (Napoleonic wars and current problems with terrorism, Iraq, Afghanistan, etc are cases in point).
"If you want peace, prepare for war" - Vegetius (I think). really, what hope is there?
Sorry for the ramble, got carried away.