"War and Peace"?

Why not just "Peace"? Why must peace always be accompanied by war? Is there a reason war exists at all?

Yes. If one were to look at mankind through the same lens we look at other species through, one could see that man is an abnormally aggressive species. Humans fight over essentially anything, from mates to food and territory. No other species claims territory as its own. It is essentially in our genetics to fight one another. From the earliest societies, man was at war with one another. It began with conflicts over food and resources. Eventually some group decided that all the food behind some line was theirs, and land disputes began.
Compare these early men to modern humans. What has changed? Not a whole lot. Most wars are still fought by groups of people claiming that the resources behind some line are theirs. Why are humans always at war with each other? Because that's the way we are.