In international relations, unlike the natural sciences, there are few fundamental principles or laws. The world map, however, reveals at least one iron law of global politics: human territoriality. Almost every inch of the globe is partitioned into exclusive and bounded spaces that “belong” to specific groups of humans. Any that is not—such as Kashmir, Jerusalem, and the South China Sea—remains hotly contested. Throughout history, territory has led to recurrent and severe conflict. States are prepared to go to war, and individuals are prepared to die, even over land with little intrinsic value. While such behavior presents a puzzle for international relations theory, a broader evolutionary perspective reveals that territorial behavior has the following three characteristics: (1) it is common across the animal kingdom, suggesting a convergent solution to a common strategic problem; (2) it is a dominant strategy in the “hawk-dove” game of evolutionary game theory (under certain well-defined conditions); and (3) it follows a strategic logic, but one calibrated to cost-benefit ratios that prevailed in our evolutionary past, not those of the present. These insights generate novel predictions about when territorial conflict is more or less likely to occur in international relations.