Neorealism – or structural realism – is a theory of international politics which is usually associated with Kenneth Waltz and his book, Theory of International Politics. The main objective of the book is to construct a very general framework for explaining recurring patterns of state behavior and state interaction in the international system. Neorealism is the bedrock theory of International Relations. Starting from a simple set of assumptions, it seeks to explain how states, in particular the most powerful ones, behave, and how they interact with each other on the international arena. It asserts that the nature of the international structure is determined by its principle of order, anarchy, and by the distribution of capabilities which is measured by the number of great powers within the international system.
The anarchic ordering principle of the international structure is decentralized, meaning there is no formal central authority; every sovereign state is formally equal in this system. These states act according to the logic of self-help, meaning states seek their own interest and will not subordinate their interest to the interests of other states. While neorealists agree that the structure of the international relations is the primary impetus in seeking security, there is disagreement among neorealist scholars as to whether states merely aim to survive or whether states want to maximize their relative power. These viewpoints are respectively of Kenneth Waltz and his defensive realism and John Mearsheimer and his offensive realism. The main purpose of this paper is to compare and contrast the theories of three neorealists thinkers, namely, Kenneth Waltz, John Mearsheimer, and Robert Gilpin to identify to what extent they differ or overlap and determine which of their theories is likely to explain better world politics. I will first identify the core argument of each author and end with the similarities and differences of their theories.
Generally speaking, the basis of Mearsheimer’s offensive realism is the core elements of Waltz’s defensive realism. The aim of Mearsheimer's (2001) theory is to explain why relations between the great powers of the modern state system are fraught with conflict. Echoing Kenneth Waltz (1979), Mearsheimer argues that the structure of international politics is key in understanding this state of affairs. Specifically, Mearsheimer (2001) relies on five core assumptions shared more or less by most contemporary realists, which characterize the essential traits of international politics. One of the assumptions is that the great powers are the main actors in world politics and international politics is played out in an anarchical realm. He claims that“it is an ordering principle, which says that the system comprises independent states that have no central authority above them. Sovereignty, in other words, inheres in states because there is no higher ruling body in the international system” (Mearsheimer, 2001, p. 30). This means that there is no 'government of governments' to enforce rules and punish perpetrators leading to anarchic system where the main purpose of states is their survival.
This view is shared by Waltz in book, Theory of International Politics (1979), that the anarchic nature of the international system encourages states to undertake defensive and moderate policies. The principle of survival is fundamental to both Waltz and Mearsheimer and the quest of power becomes the ultimate goal of states. Power plays a key role in Mearsheimer's thinking. Like most contemporary realists he follows Waltz (1979) and equates power not with outcomes but with material capabilities (Mearsheimer 2001), arguing that outcomes are a poor predictor of the balance of power since weak states may sometimes defeat their stronger opponents (2001). Unfortunately, measuring capabilities reliably is difficult and realists have hotly debated this issue. In The Tragedy of Great Power Politics, Mearsheimer contributes to the realist power debate by going to great lengths to clarify his views on the subject. Dividing state capabilities into latent (economy and population) and actual power (military), Mearsheimer argues that military power is most important when analyzing world politics because it is 'the ultimo ratio of international politics' (2001).

Mearsheimer’s definition of the international structure is in accordance with the modern realist research programme as outlined by Kenneth Waltz (1979). However, two of his assumptions are contrary to Waltz’s viewpoints. First, although, shared by most contemporary realist scholars, the rationality-assumption is controversial. For instance, Waltz argues that structural realism does not need any distinct assumption of rationality (Waltz 1979) although, he does admit, that states are rational in the sense that they are sensitive to costs. A generally accepted and important qualifier on the rationality-assumption is however, that the states chose the most efficient strategies on the background of imperfect knowledge. Second, Mearsheimer’s assumption that all states posses some offensive military capability is not generally regarded as a core assumption of Waltz’s programme. Furthermore, it is difficult to include such an assumption into a definition of international structure. This is because offensive military capability is a specific unit-level attribute associated with the individual states and not with the relations among them. However, even though Mearsheimer makes no explicit attempt to do so, one could argue that offensive military capability is a structural trait because even though military capability is a unit attribute the distribution of military capabilities across units is not.
Mearsheimers depiction of the international system as an arena of relentless competition for power clearly distinguishes his approach from Waltz's more benign version. Rather, the nature of Mearsheimer's pessimistic analysis resembles more closely Hans Morgenthau (1948). Mearsheimer also departs from Waltzian neorealism in another important respect by including an intermediary variable at the unit-level, that is, state location, into his theory of offensive realism. By doing so, he makes a clear break with Waltz's purely structural theory in which the international distribution of power capabilities is the only variable of consequence (1979). The inclusion of geopolitics, however, is far from alien to the broader realist tradition. In fact, geopolitics has been present in realist thought for centuries and geography also plays a prominent role in recent, so-called post-classical realist contributions that in addition to the international power structure take into account the effects of non-structural systemic variables.
By including state location in addition to the distribution of power, Mearsheimer also defies Waltz's curious analytical demarcation between theories of international politics and theories of foreign policy. Mearsheimer's theory explicitly tells us both something about aggregate international outcomes ⎯ for example, the frequency of wars and patterns of alliance formation ⎯ while at the same time it also provides us with explanations of individual state behaviour. This contrasts Waltz's extremely sceptical position concerning the possibility of theorizing about foreign policy because of the complex nature of variables at the unit-level and his firm rejection that his own structural theory has much to say about foreign policy at all (Waltz 1979). By explicitly taking structural realism into the sphere of foreign policy, Mearsheimer's offensive realism, thus, moves away from his otherwise Waltzian point of departure and towards a body of recent realist theories of foreign policy ⎯ the so-called neoclassical realism.
In essence, however, structure does most of the explaining in Mearsheimers' theoretical universe and his theorizing stays firmly within the structural realist mould. For instance, whereas the 'evil school' realists identified the causes of international power struggles within the corrupt nature of man or within the nature of 'bad' states, Mearsheimer, like Waltz, insists that causes at the structural level rather than the unit-level are the primary key to understanding world politics. Moreover, although location plays an important role in Mearsheimer's thinking this variable is wholly subordinate to the structural balance of power variable since different power constellations determine the impact of location on state behaviour. For instance, in Mearsheimer's theory geography tells us when and which states are likely to balance or buck-pass. However, it does not affect the basic structural incentives to maximize relative power. This represents a distinct difference between Mearsheimer's thinking and the recent writings of Kenneth Waltz who tend to relegate independent effects to non-structural variables. Thus, although Mearsheimer straddles insights from different strands within the broader realist tradition, he ultimately devotes primary importance to the effects of the international structure.
Robert Gilpin shares the core assumptions of modern realism, but his study of The Theory of Hegemonic War also attempts to cope with some of the criticism leveled at Waltz’s theory by focusing on the dynamics of system change. In doing so, Gilpin also seeks to avoid the criticism that the Waltz theory is largely ahistorical. Drawing upon both economic and sociological theory, his model is based on five core propositions. The first is that the international system is in a state of equilibrium if no state believes that it is profitable to attempt to change it. Second, a state will attempt to change the status quo of the international system if the expected benefits outweigh the costs. Related to this is the proposition that a state will seek change through territorial, political, and economic expansion until the marginal costs of further change equal or exceed the marginal benefits. Moreover, when equilibrium between the costs and benefits of further change and expansion is reached, the economic costs of maintaining the status quo (expenditures for military forces, support for allies, etc.) tend to rise faster than the resources needed to do so.
An equilibrium exists when no powerful state believes that a change in the system would yield additional net benefits. Finally, if the resulting disequilibrium between the existing governance of the international system and the redistribution of power is not resolved, the system will be changed and a new equilibrium reflecting the distribution of relative capabilities will be established. Unlike Waltz, Gilpin includes state-level processes in order to explain change. Differential economic growth rates among nations -a structural-systemic level variable - play a vital role in his explanation for the rise and decline of great powers, but his model also includes propositions about the law of diminishing returns on investments, the impact of affluence on martial spirit and on the ratio of consumption to investment, and structural change in the economy.

In a nutshell, although I am not supportive of Mearsheimer’s offensive realism, I would assume that it is the theory that account best the today’s world politics. Firstly, it can be said that Mearsheimer’s offensive neorealism provides an alernative complement to Waltz’s defensive neorealism. The theory adds to defensive neorealists’ argument that the structure of the international system constrains state behavior. Setting to rectify the status quo bias pertaining to defensive neorealism by arguing that anarchy can also generate incentives for states to maximize their share of power, offensive neorealism solves anomalies that Waltz’s theory fails to explain. Mainly, the theory is able to provide an explanation for the amount of conflict occurring among states in the international system.

Moreover, this complementarity could signify theoretical interrelation with the two theories working in alternation to explain state behavior, thereby allowing for a more complete structural realist theory that can more accurately account for both defensive and offensive state behavior. Secondly, these scholars uphold the argument that Mearsheimer’s offensive neorealism significantly contributes to foreign policy theory and alliance theory. More specifically, Mearsheimer’s theory goes a step further than structural defensive realism by successfully theorising both international politics and foreign policy. Contrary to Waltz’s rejection of defensive neorealism as a theory capable of explaining foreign policy on top of international politics, offensive neorealism includes explanations of both international outcomes pertaining to the systemic level of analysis and individual state behavior. Additionally, the inclusion of new variables such as geography alongside the distribution of power enhances offensive neorealism’s potential to make specific assumptions about states’ pursuit of aggressive actions and resort to balancing and buck-passing.