As some of said, we all gotta face it, people are assholes. Simple as that, world peace will never happen because people's greed drives them to gain power. But on the other hand, in the US's example, if we just stopped all war now (like the hippies want us to) then who here thinks that our enemies will just lay down their guns and sing Kumbaya and we will skip off to never ever land, noone, because we know that if we stopped, they would just rebuild their forces, and more tragedies like 911 would happen.