Though not a single shot is fired in the novel, A Separate Peace can be thought of as a war novel. World War II is a looming presence that none of the boys at Devon can escape. Once they graduate, they'll have to enlist. This fact makes the separation between childhood and the adult world very clear. Childhood is the high school world of sports, dreams, and carnivals, while the adult world is one of war. And while many of the boys early in the novel, lost in a haze of childhood innocence, yearn to fight in the war, by the end of the novel they realize that the war and the adult world is full of hypocritical hatred and "honor." In fact, the first boy who does enlist, Leper, literally goes insane from what he finds.
Yet A Separate Peace is focused more closely on a war between individuals, a rivalry. This is a personal war of competing egos (or one competing ego), in which Gene's rivalry with his best friend Finny results in Finny's tragic accident, and then his tragic death. Through this personal battle, A Separate Peace shows the internal war people fight in making the transition from the "separate peace" of teenage innocence to the harsh realities of adult life. It also draws a parallel between the forces that motivate personal rivalries and the forces that result in World Wars, suggesting that both arise from the same flaws of enmity and jealousy in every human heart.