Modern states emerged from a crucible of economic, military, and institutional competition. They proceeded out of western Europe to directly and indirectly dominate all other regions of the world. War proved central to this transition; however, historians and social scientists remain undecided whether war itself drove the development of the modern state and the modern states system. Proponents argue that as several types of political organization arose in the late medieval period, those best able to collect resources and deploy them for war prevailed. In a process of natural selection, then, political entities either adapted certain characteristics or ceased to exist. If true, this claim carries important implications for the modern states system. Specifically, like animals bred for aggressiveness, the central units of international politics ...