Realism has been one of most prominent and dominant theory of international relations since the conception of the discipline. It relies upon an ancient tradition of thought which includes writers such as Thucydides, Machiavelli and Hobbes. There are various and differentiated features of modern realist thinking. However, the main principles of the theory (such as statism, survival, and self-help) remain a commonality among all the realist variations. This essay aims at comparing two prominent authors of the field, Kenneth Waltz and Robert Gilpin, in order to systematically allow for their most important contributions to be highlighted. How are they similar? How do they differ? And most importantly, how did they revolutionize the International Relations field? These are some of the questions which will be tackled. When Robert Gilpin published War and Change in World Politics almost four decades ago, Realism was headed for a major revival1. Given the importance of this scholarly tradition and the degree to which other schools of thought developed in response to it, how realism ended up revivifying and modernizing itself would have profound consequences for the International Relations discipline as a whole. Where would the scholarly field of international relations be today if Robert Gilpin had become the standard bearer for realism instead of Kenneth Waltz?2 This is a question worth asking. Robert Gilpin, though one of the influential thinkers in the international relations scholarship of the past half century, remains well below that of his contemporary Kenneth Waltz, due to many reasons. In a nutshell, it can be argued that the international relations ‘paradigms’ got defined in a manner that obscured Gilpin’s contribution. Following and boosting an already established trend, Robert Keohane’s work Neorealism and its Critics portrayed Waltz, not Gilpin, as definitive of contemporary realism and as the preferred base for the development of scholarship. Then, Waltz’s work thus came to trump all others as the definitive modern restatement of realism. And because realism plays such a large role in IR ? if only as foil for others’ work ? whoever came to be seen as definitive of that approach and whoever came to be seen as offering the main alternatives to it would have an outsized influence on scholarship. And so it happened. Realism continued to develop in Waltz’s shadow. Why did Waltz’s Theory of International Politics come to be seen as the definitive work of modern realism, rather than Gilpin’s War and Change? The more one compares the two works, the more puzzling their respective roles in the field become.On one hand, both authors are deeply learned. Both are undeniably realist, since their theorizing follows and considers the core beliefs of the realist school (even though Waltz’s theory was actually neo-Realism). Besides, both are grounded in classical and modern works and are firm within the state-centric tradition. Gilpin’s view, like that of Waltz, also seeks an accommodation with structural theory. It is interesting to think how both titles are actually misleading: Waltz’s book is not really a theory of international politics. It does not address in any explicit way most of the phenomena that are encompassed by that term. Rather, Waltz presented a theory that intended to help answer a few important but highly general questions about international politics, such as why the modern states system has persisted in the face of attempts by certain states at dominance; the recurrence of balances of power; why war among great powers recurred over centuries; and why states often find cooperation hard. In addition, the book forwarded one more specific theory: that great-power war would tend to be more frequent in multipolarity than bipolarity. Gilpin’s book is no less all-encompassing and addresses a set of questions no less central to both the realist tradition and IR more generally: how to explain change in international politics; why defined international orders rise and decline; the causes of great wars and long periods of peace; and the rise and decline of hegemonic great powers. It could be argued that War and Change actually yields more relevant, testable middle-range theories than Waltz’s book. On the other hand, there are several differences between the theorizing of both authors. The apparent relevance of the books to the events of the day can be considered as an important one. At the time, neither ‘war’ nor ‘change’ seemed to resonate with what seemed like a stable Cold War stalemate. By contrast, Theory of International Politics stressed the enduring verities of international relations in general and the Cold War in particular. He argued that the principal change in the international system over several centuries was a shift to bipolarity after WWII, and stressed “unit-level” processes. To him, the “units” in question are states “whose interactions form the structure of international political systems”, and which are, in his opinion, functionally identical. The deep anarchic structure of world politics meant that constrained rivalry and cooperation between the United States and the Soviet Union would remain the central issue. Another important difference between both authors is the fact that Waltz presented his arguments in a way that best fitted the particular conception of social science that was just becoming fashionable among American political scientists. This was the idea that the great scholarly traditions of IR such as Realism and Liberalism should be refashioned as internally coherent scientific research programs comprising a hard core of assumptions and a related set of scope conditions and specific propositions. In other words, Waltz’s book seemed tailor-made for it while Gilpin’s did not. The books’ very titles give a hint: one is about the specific problem of change, while the other intents to be a comprehensive ‘theory of international politics’. Thus, the latter is more likely to be seen as the reformulation of realism into a scientific research program. In regard to the historical background analyzed, it can be said that War and Change has an advantage over Waltz’s Theory. While Waltz’s empirical references were almost solely confined to the post 17th century European international system and its global successor, Gilpin’s analysis included pre- and non-European international systems stretching back to antiquity. Gilpin’s book was also more comprehensive than Waltz’s in its focus on the interaction between economics and politics. He claimed ‘to provide a framework for thinking about the problem of war and change in word politics’, trying to provide a better theoretical framework for understanding war and change. But, Waltz’s Theory provided a far more attractive and convenient complement for other scholars because it possessed many attractive features that War and Change lacked.. Theory was more thoroughly ‘structural’, operating solely at the systemic level. War and Change created a platform which allowed for interaction between the domestic and systemic levels. Theory appeared to understand structure as material, while War and Change appeared to give an important roles for ideas. Thus, Waltz’s neo-Realism is arguably less concerned with change than that Gilpin’s Realist framework of analysis. It might be said that Theory of International Relations was written in such a way as to make many things about international relations seem to be major theoretical puzzles in need of scholarly explanations. The theory could be read as saying, for example, that domestic institutions do not matter in explaining large-scale patterns of war and peace. Any theoretical or empirical demonstration that the nature of domestic institutions ‘matters’ in accounting for these patterns could be considered as a major finding. The same phenomenon does not happen to Gilpin’s writings. He manages to say new and non-obvious things about international politics without seeming to deny the possibility of such a large number of easily observable facts of international life. Gilpin’s work does not rule out a causal role for ideas, institutions and domestic politics, but rather stress their interaction with material power. Even though they are both clearly realist works, the two books are built on very different foundational assumptions12. Waltz’s theoretical edifice rests on the assumption that states are conditioned by the mere possibility of conflict, while Gilpin assumes ? more in keeping with expected utility theory and most mainstream social science ? that states make decisions based on the probability of conflict. Waltz’s worst-case, possibilistic assumption was the key link between the condition of anarchy and all the strong and counterintuitive implications about state behavior. In Gilpin’s probabilistic world, however, states may well choose a wide variety of strategies depending on their assessment of the probability and severity of security threats. They may choose to pursue economic gain instead of security if the probability of conflict is low, or they may choose to pursue power and prestige in the near term in order to be more secure in the long term. Thus, for Gilpin, states do not always ‘maximize security’ at all times and under all conditions, as Waltz held. Theories that ‘assume that one can speak of a hierarchy of state objectives … misrepresent the behavior and decision-making of states’, he insists. Rather, ‘it is the mix and tradeoffs of objectives rather than their ordering that are critical to an understanding of foreign policy’. A last considerable difference between both concerns balance-of-power theory. Waltz’s work is built upon a rigorous rendering of this venerable theory, which he posited as the ‘realist’ explanation for the ‘recurrence of balance’ through international history, by which he meant the failure of repeated bids for European or global hegemony. Gilpin presented a completely different theory to explain the same phenomenon as Waltz. Surveying a far broader sweep of history, Gilpin argued that the balance of power played a distinctly secondary role in limiting hegemonic expansion when compared to other countervailing forces such as natural barriers and the loss-of-strength gradient, economic and technological limits to optimal size, and domestic institutions. Unlike the ‘apples vs oranges’ criticisms of Waltz’s balancing theory, which tended to focus on short time spans or a few cases, this was a proposition directly contradicting Waltz, for it explained an similarly general empirical regularity over a very broad sweep of history. In conclusion, had IR not been tricked by the idea that grand traditions of scholarship like realism and liberalism had to be translated into single theoretical ‘research programmes’, there is every likelihood that Gilpin’s seminal treatment of war and change would have been recognized as being every bit as definitive a restatement of realist theory as Waltz’s treatment of balance-of-power theory. And had that occurred, international relations research might have unfolded quite differently. It might be argued that War and Change never directly addressed the scenario that was about to unfold: the decline of a clearly weaker challenger to a given hegemonic order. In keeping with the mood of the time, the book treats the Soviet Union as the main and most dangerous rising challenge to the United States, which is portrayed as in serious relative decline. It is hard to read the book today and avoid to think that, had readers at the time known that it was the Soviet Union, not the United States, that was in steep relative decline, their eyes would have been opened to the likelihood of peaceful change.