In Blood Rites: Origins and History of the Passions of War, Barbara Ehrenreich explores the emotions and psychology of warfare.

She starts with an aspect of evolutionary biology that I hadn't encountered before. During early stages of human development, man was, like other primates, the prey of stronger and more powerful creatures. Avoiding those beasts and banding together to challenge them became a deeply entrenched part of human psychology. Some vestiges of those early experiences remain within us, such as the nightmares children have about being attacked by animals and the thrills of horror movies in which monsters prey on people.

Eventually, people gained the abilities and tools necessary to become predators instead of prey. However after the animal threat weakened, the underlying emotions shifted their focus to sacrifices (both animal and human) and war. The evolutionary background gives warfare the significance, fervor, and flavor of a religion.

Ehrenreich traces how attitudes about war evolved throughout history. At first, all men in the tribe acted as warriors. In some cultures, in fact, fighting in battle was a rite of passage to become a man. Later on, after various technological improvements, warrior elites arose, such as knights in Europe and Japanese samurai. Those elites fought and dominated the social structure, while the masses provided supplies and support. With the development of guns and the formation of nation-states, military forces became large bureaucratic institutions driven by strong nationalistic forces.

Blood Rites in an excellent study of warfare that includes many innovative ideas supported by strong arguments.