IR scholars has long debated on which level of analysis is the most appropriate and helpful level in approaching international relations. In his seminal book Man, the State, and War, Kenneth N. Waltz becomes the first to analyze the political philosophy behind each level of analysis and their interaction with one another. Unlike his later writings in which he develops a purely structural theory of international relations, in Man, the State, and War Waltz offers a more balanced view on the importance of each "images" for the study of world politics: "the `third image' describes the framework of world politics, but without the first of and second images there can be no knowledge of the forces that determine policy," (238).
To briefly summarize Waltz's images, the "first image" is about human nature. Human-nature accounts explain war by analyzing the common characteristics (or defects) of human beings. These theories tend to attribute war to an "ultimate cause" that derives from human nature: "the root of all evil is man, and thus he is himself the root of the specific evil, war," (3). Waltz's problem with searching for an "ultimate cause," however, is that ultimate causes frequently turn out to be the cause of everything. Therefore, Waltz criticized theories that explain war through human nature by arguing that human nature is the cause of as many good (and benign) things as evil ones (39).
The "second image" is about the characteristics of states: "the idea that defects in states cause wars among them," (83). Waltz analyzes several state-level accounts of war and peace some of which are very fashionable today, such as the peaceful nature of democracies and peaceful impact of free trade. The notion here is similar to the first image, if "bad" states (such as non-democratic or interventionist) can be erased then there will be no war (119). However, Waltz notes that there is no guarantee that good states will not revert to war. Waltz rejects state-level theories that would rely "on the generalization of one pattern of state and society to bring peace to the world," (122).
The "third image" is the international system. The absence of a world government renders the international system an anarchical one; and "in anarchy there is automatic harmony," (160). Thus, wars occur "because there is nothing to prevent them," (232). Waltz tends to view third image as the most important account of war among nations. Yet unlike in his later theorizing, he underlines the importance of the other two images: "we still have to look to motivation and circumstance in order to explain individual acts," (231). Hence, multiple levels of analysis.
My personal view on the levels of analysis question is that among the three different levels from which IR scholars approached to the study of conflict among states, state-level approach has been the most productive and helpful in terms of accounting for the conflict among states and providing us clues as to how to reduce or manage them. Thus, I do not share Waltz's inclination to the third image in Man, the State, and War. Yet in the final analysis, any single level is incomplete by itself. Waltz's Man, the State, and War is important for being the first to analyze the philosophical foundations of each levels of analysis and to argue the complementary relationship among them. "The real problem of IR scholars," Lipson observed, is "to integrate choice and structure," (1884, p. 20). And a successful integration of choice and structure inevitably requires making use of systemic as well as state-level theories. Indeed, this is the current trend in both theoretical approaches (Moravcsik 1997; Gilpin 2001) to and empirical analyses (Huth 1996) of international relations.