Kenneth Waltz’s Theory of International Politics created a “scientific revolution” in international relations, starting two major debates. In the 1980s it defined the controversy between the neorealists, who believed that competition between states was inevitable, and the neoliberals, who believed that states could co-operate with each other. As the debate wound down with the end of the Cold War in the 1990s, a more fundamental, second debate began: Is it possible to treat international relations as a science?

Waltz aimed to answer the question: “If changes in international outcomes are linked directly to changes in actors, how can one account for similarities of outcome that persist or recur even as actors vary?” Taking a unique approach, Waltz did not look at any one unit within the system—avoiding discussions of any particular state or political leader. Instead, he examined the system itself.