War is the ultimate expression of violence. Violence is the ultimate expression of force. Force is the ultimate expression of power. And power, people, is the ultimate key to survival.
So, in creatures complex enough to do it, war is a natural instinct. Well, it's a natural instinct to those who benefit, yet are not risked. Politicians, leaders, the assholes that are getting fat off it, yet not going to potentially die during it. War is a bastardized hybrid of natural instincts and the complexity of human logic.
Also, to hell with your "conservation" theory. If people really wanted to, really tried, we could support the entire human population in within the borders of Texas. Feed, clothe, and house, all 6-billion plus, without ever having a human body leave that state.
