When Kenneth Waltz passed away on 12 May 2013, International Relations (IR) lost one of its finest theorists. Waltz contributed not just one, but three major works to the field – Man, the State and War (1959), Theory of International Politics (1979) and The Spread of Nuclear Weapons: More May be Better (1981) – as well as many that are less well known, including Foreign Policy and Democratic Politics (1967). His prodigious achievements in research were matched by those in education: at Berkeley and at Columbia, he nurtured the talents of a series of prominent scholars, including Stephen Van Evera, James Fearon, Christopher Layne, Barry Posen and Stephen Walt. But Waltz’s greatest legacy to IR was his revival – indeed, his resurrection – of realism.
The classical realists of the 1940s and 1950s have long been celebrated for the pioneering efforts and the subtlety of their analysis of international politics. Together, W. T. R. Fox, Walter Lippmann, John Herz, George Kennan, Hans J. Morgenthau, Nicholas Spykman, and the rest delivered a much-needed corrective to the optimistic and legalistic tendencies of liberal internationalism. And they furnished instinctively liberal and internationalist American policy-makers with a crib-sheet of how to conduct power politics in the atomic age, helping to craft ‘containment’ and to restrain those who wanted to ‘roll-back’ communism, even if it meant fighting another devastating war.
The classical realists did not, however, have things all their own way. They were isolated from the mainstream of American social science, which tended, then as now, to be progressivist and scientistic. Their mostly Central European accents and dark prose did not help. They particularly disliked behaviouralism, which flourished in the 1950s, and which aspired to a theory of human social behaviour in all spheres of activity. They tried, in response, to argue that the study of IR was distinctive, not reducible even to political science, and impossible to grasp in conventional, social scientific terms.
What they offered instead was an account of international politics which flowed from a highly contestable philosophical anthropology drawn either from the Judeo-Christian tradition or from contemporary German philosophy, especially Nietzsche, or a combination of both. ‘Man’, they argued, was a flawed beast, prone to sin or to what Morgenthau called an animus dominandi. Both could be kept in check by laws and the robust enforcement of them, but they could never be eradicated from human nature. The fewer the laws and the weaker the means of enforcement, the more these tendencies could shape the behaviour of leaders and their followers. This is what made international – as opposed to domestic politics – so challenging, argued the classical realists, because in that realm these checks on human nature were to all intents and purposes non-existent. The only thing that could be used to deal with the animus dominandi was power, military, political or economic, to be wielded, when necessary, unflinchingly and without scruple. The object of IR theory, as the classical realists saw it, was thus to educate leaders about ‘man’ and about ‘power’ by summarising the collected wisdom of European statecraft.
Waltz recognised early on that classical realism was problematic. In particular, the philosophical anthropology on which it was based was impossible to prove or disprove – it rested simply on inherited beliefs about human nature that, to his mind, lacked scientific credibility. The ‘evidence’ on which it was based was not a systematic body of evidence and analysis, but rather sets of religious or philosophical speculations. As such, Waltz was convinced, it would not stand as a theory.
Waltz signalled his discomfort with the underpinnings of classical realism in Man, the State and War, but it was only in Theory of International Politics, published two decades later, that he set out the alternative. In the meantime, others had also rounded on the realists, asking similar questions as Waltz about its underpinnings and its plausibility, but supplying rather different accounts of what international theory should look like. In the 1960s and 70s, the classical realists were hounded by behaviouralists, systems analysts, game theorists, neo-functionalists and institutionalists, and pushed out of the theoretical mainstream. Thinkers like Morton Kaplan, Anatol Rapoport, Ernst Haas, Joseph Nye and Robert O. Keohane came to the fore, displacing the realists and shifting the core concerns of the field away from issues of human nature and power politics.
What Waltz did in this context was remarkable: almost single-handedly, he resuscitated realism, amputating those parts that were clearly dysfunctional, giving it the transfusions of new thinking that it needed, and returning it, revivified, to the fray.
Theory of International Politics shifted realism away from metaphysical speculation on human nature and onto firmer ground by removing any need for a philosophical anthropology to explain why international relations are as they are. Instead of a contentious account of ‘man’, Waltz substituted a structural account of the international system that borrowed heavily from the theory of the firm in classical economics.
Waltz’s structural realism attracted criticism from the start, and continues to do so today, almost twenty-five years after Theory of International Politics was published. But it is impossible not to acknowledge that it decisively shifted the terms of debate in international theory, returning realism to the mainstream, where it has remained ever since. In the 1980s, 1990s and 2000s, the field was defined by a series of arguments between the realists and their critics, as first the neo-liberal institutionalists and then various bands of constructivists, feminists, postmodernists and critical theorists lined up to attack Waltz and his students.
Without Waltz and without structural realism, we would have seen no ‘offensive’ and ‘neo-classical’ realism, no ‘agent-structure’ debate, and no ‘anarchy is what states make of it’. Whatever one thinks about his revival of realism, and about the many responses to it, it is impossible to imagine what IR would have looked like without Theory of International Politics, as well as Waltz’s many other works. For that reason alone, he will be remembered as one of the great thinkers of the field.
Ian Hall is a Senior Fellow and Director of the Graduate Studies in International Affairs program at the Australian National University. He has published on the intellectual history of International Relations and on Indian foreign and security policy. His latest book is ‘Dilemmas of Decline: British Intellectuals and World Politics, 1945-75′ (University of California Press, 2012).
 For memories of Waltz as a teacher, see inter alia Robert Art and Robert Jervis, ‘Kenneth Waltz and his Legacy: The Man and the State of War’, Foreign Affairs, 22 May 2013, at: http://www.foreignaffairs.com/articles/139400/robert-art-and-robert-jervis/kenneth-waltz-and-his-legacy, Stephen M. Walt’s obituary, ‘Kenneth N. Waltz, 1924-2013’, Foreign Policy, 13 May 2013, at: http://walt.foreignpolicy.com/posts/2013/05/13/kenneth_n_waltz_1924_2013, and Mary Zhou, ‘Kenneth Waltz, principal theorist in international relations, dies at 88’, The Daily Californian, 29 May 2013, at: http://www.dailycal.org/2013/05/29/kenneth-waltz-principal-theorist-in-international-relations-dies-at-88/.
 There many, many books and articles that cover this ground, but see especially Campbell Craig, Glimmer of a New Leviathan: Total War in the Realism of Niebuhr, Morgenthau, and Waltz (New York: Columbia University Press, 2003) and Joel H. Rosenthal, Righteous Realists: Political Realism, Responsible Power, and American Culture in the Nuclear Age (Baton Rouge: Louisiana State University Press, 1991).
 This story is told, and told well, in Nicholas Guilhot (ed.) The Invention of International Relations Theory: Realism, the Rockefeller Foundation, and the 1954 Conference on Theory (New York: Columbia University Press, 2011).
 Hans J. Morgenthau, Scientific Man vs. Power Politics (Chicago: University of Chicago Press, 1946), p. 193. See also Annette Freyberg-Inan, What Moves Man: The Realist Theory of International Relations and its Judgment of Human Nature (Albany: SUNY Press, 2004).
 See Kenneth N. Waltz, ‘Realist Thought and Neorealist Theory’ (1990), in his Realism and International Relations (London and New York: Routledge, 2008), pp. 67-82.