This essay argues that the history of the international system has revolved around a moving frontier of cultural exclusivity. It is one of a series of working papers commissioned by the World Models Project in its effort to stimulate research, education, dialogue, and political action aimed at contributing to a movement for a just world order. Originating under monotheism, the cultural frontier has been characterized by a persistent "us/them" dichotomy. Civilizations which anthropomorphized God in monarchical terms tended to divide the world between the God-fearing and sinner. This tendency was reinforced by the culture of politics which differentiated supports from adversaries. Both were embodied in early international law such that a system of rules for civilized nations did not apply to "them"--the rest of the world--thus opening the door to imperialism and eventual class stratification in the international system. Although the cultural frontier has been moving due to secular challenges, the major challenges to Judaeo-Christian monotheism--Marxism and Islam--are themselves dualistic: the Marxist dialectic is inherently of this nature as is the tension between good and evil in Islam. The interrelationship between major cultural themes in today's world, coupled with a developmental system of stratification which is based on technical know-how, suggests that important but hidden problems of a cultural nature are contained in the world order agenda. (Author/RM)