The subject of early warfare is controversial: Some authors argue that it has been prevalent throughout human biological evolution while others argue that it arose more recently. We study warfare over land among internally egalitarian groups, which were the norm for most of human prehistory. Archaeological evidence for Europe and southwest Asia indicates that warfare was rare in the Upper Paleolithic, common in the Mesolithic, and widespread in the Neolithic. This suggests an increase in the frequency of warfare along the trajectory from mobile to sedentary foraging, and from sedentary foraging to agriculture. We constructs a model in which two groups occupy sites with possibly different productivities, and each group must decide whether to attack the other. If either group attacks, the probability of one group seizing the land of the other depends on the sizes of the two populations. If neither attacks, there is peace. We show that when individual agents can freely migrate between sites before group decisions about warfare are made, a stable equilibrium with warfare cannot occur. However, a model involving costly individual migration and Malthusian population dynamics can generate warfare if climatic or technological shocks alter the relative productivities of the sites.

A large body of archaeological and anthropological research suggests that warfare is more common when societies are stratified. This is true for societies based on either sedentary foraging or agriculture. We argue that warfare in stratified societies does not require climatic or technological shocks, and results from competition among rival elites over land rent. In our model, elites recruit specialized warriors by offering booty in the event of victory, which may involve elevation to elite status. After each elite recruits an army, the rival elites must decide whether to attack, defend, or flee. We solve for the equilibrium at the combat stage as a function of army sizes, and use backward induction to solve for the equilibrium army sizes. If stratification is relatively low (the land rents are small relative to commoner food income), elites can sometimes win through intimidation without fighting an actual war. But if stratification is high, such equilibria disappear and the only outcome is a mixed-strategy equilbrium with a positive probability of open war. In either case, successful elites expand their territory. Fiscal constraints on the capacity of elites to recruit warriors can sometimes limit warfare, but do not prevent it entirely.