Kenneth Waltz made critical contributions to the development of international relations theory, the debate on U.S. foreign and security policy, and the training of a new generation of international relations scholars. In the course of these contributions he has also served as a model of clarity of expression and logical argument.
Waltz's first critical contribution to international relations theory was his elucidation of the "levels of analysis" problem in Man, the State, and War. The categorization of explanatory theories into ones that respectively stress the role of human nature; state-level political, economic, and social systems; or the anarchical condition of international politics has proven a critical building block of modern international relations theory.
Through his exploration of theories at different levels of analysis, Waltz took the first giant step of his theoretical career. Even readers who do not wish to take the same step find the path illuminating. Waltz poses two major questions: If bad humans are the cause of war, why is there so much peace? If bad states (or societies, or economies) are the cause of war, why are so many good states implicated in wars? Why does war persist despite variation in the nature of the states and societies that make up the international system? Waltz suspected that classical realism and balance of power theory would tell us something about this -- but at that point he could not quite put his finger on it.
The device of "levels of analysis" has become a ready element of our vocabulary about international relations theories. It travels well. Even novices quickly grasp its utility, and find themselves better able to organize both their own and others' arguments after a brief introduction. More importantly, Man, the State, and War makes the case that there must be something distinctly "international" about international politics, which Waltz suspected could be traced to the absence of a sovereign. The book maps out a space that subsequent theorists, including Waltz, would then need to fill. After we read the book, we know this. Some may disagree with Waltz's subsequent arguments about the share of international politics that can be explained by "third image" -- or system-level -- theories, but none can dismiss them.
Waltz's second contribution to international relations theory is to be found in Theory of International Politics. There, Waltz distills the product of years of thinking about the "third image." He develops the distinctive concept of "international structure" as the distinguishing feature of the international political system. Structure is the minimalist device that captures the basic causal forces and variables of the international political system. Structure is made up of an organizing principle (anarchy) and the distribution of capabilities (polarity). From that, Waltz deduces many of the characteristic patterns of behavior one should find in the relations among autonomous actors. Though Waltz overlooked certain possibilities, such as unipolarity, and left much room for argument about some of his specific deductions, one cannot come away from Theory without a keen appreciation of the constraints and incentives imposed upon states by the condition of anarchy. These lead to the competitive behaviors that are so familiar.
Kenneth Waltz was one of the most influential international relations scholars of the last half-century. Even if he had produced no students, his books would stand as rocks in the road. Nobody trying to understand international politics can avoid them. But Kenneth Waltz did produce many students, who built on his work in their own scholarship, further refining the realist model. Even international relations theories based on other premises must engage with structural realism. Kenneth Waltz found what was international about international politics and built the foundations of the field.