ABSTRACT
True, if the conditions that a theory contemplated have changed, the theory no longer applies. But what sorts of changes would alter the international political system so profoundly that old ways of thinking would no longer be relevant? Changes of the system would do it; changes in the system would not. Within-system changes take place all the time, some important, some not. Big changes in the means of transportation, communication, and war fighting, for example, strongly affect how states and other agents interact. Such changes occur at the unit level. In modern history, or perhaps in all of history, the introduction of nuclear weaponry was the greatest of such changes. Yet in the nuclear era, international politics remains a self-help arena. Nuclear weapons decisively change how some states provide for their own and possibly for others' security; but nuclear weapons have not altered the anarchic structure of the international political system.
