The psychology of Nations, by G.E. Partridge

The simplest possible interpretation of the causes of war that might be offered is that war is a natural relation between original herds or groups of men, inspired by the predatory instinct or by some other instinct of the herd. To explain war, then, one need only refer to this instinct as final, or at most account for the origin and genesis of the instinct in question in the animal world. Some writers express this very view, calling war an expression of an instinct or of several instincts; others find different or more complex beginnings of war.