While complete peace is not achievable in this fallen world, the only way to move towards that direction is for everyone to have the same idea of peace. All countries and their citizens will never completely agree upon this, however, as this world is full of binaries and linear thinking of the human inhabitants.
Obviously, the world working together would be more beneficial. We could actually develop systems to protect us from external threats (buster asteroids, etc) and we'd stop killing and hurting each other here at home. It's not feasible, there are far too many factions disinterested in working together, but in a perfect world we'd be a lot further along if we stopped hating each other.
I think that humans should all try to make world peace a possibility. Even though I do not think it is possible, it doesn't mean people shouldn't try. I think that the act of trying to strive for world peace would make the planet a better place regardless even if it is not attained.
The definition of world peace is purely subjective. You will get a different answer from different people based on the part of the world they reside in and their overall upbringing. If we could all come together on the basic foundation of what world peace means and what it takes to achieve, maybe then we could all focus on the common goal. But for not, it's not that simple.
For one, world peace is not possible. And, secondly, there are so many differing ideas on what world peace should be. For some, it's the ability to do whatever they want, while being left alone. For others, it's a huge "nany states." And, for others such as myself, I like a little bit of danger.