In Ride of the Second Horseman, Robert O'Connell probes the distant human past to show how and why war arose. He begins with a definition that distinguishes between war and mere feuding: war involves group rather than individual issues, political or economic goals, and direction by some governmental structure, carried out with the intention of lasting results. With this definition, he finds that ants are the only other creatures that conduct it - battling other colonies for territory and slaves.

But ants, unlike humans, are driven by their genes; in humans, changes in our culture and subsistence patterns, not our genetic hardware, brought the rise of organized warfare. O'Connell draws on anthropology and archeology to locate the rise of war sometime after the human transition from nomadic hunting and gathering to agriculture, when society split between farmers and pastoralists.