Against this background of growing disenchantment with realism, John Mearsheimer, one of its most distinguished and articulate proponents, offers a spirited and energetic defense of the theory. The point of departure is similar to Kenneth Waltz's (Theory of International Politics, 1979): Whether or not states are by nature power hungry, they have no choice but to act as if they were. In an anarchic international system (defined by the absence of supranational government), all states suffer permanent insecurity. Uncertain about the intentions of others, each must assume that their purposes are predatory. Under the circumstances, all states acquire a military capacity that, in most instances, has offensive applications, simply to counter that of others. Here, Mearsheimer parts company with structural realism, which assumes that security is sought in an international balance of power, one that stands to ensure the status quo. That variant of realism, characterized as defensive realism, does not, says Mearsheimer, reflect the actual calculations of states. In fact, there are virtually no status quo powers; rather, states act offensively to acquire more power than others; thus, a theory of offensive realism better captures the security dilemma. There are, Mearsheimer points out, limits to how much power a state can acquire. Global hegemony is not possible because of the difficulty of projecting power across large bodies of water. The ultimate achievement is to become a regional hegemon—while discouraging the emergence of hegemons in other regions. In this regard, the least stable arrangement is a bipolar distribution of power; a balanced multipolar system is not as desirable, but it is preferable to unbalanced multipolarity.

The thesis is lucidly presented and argued with verve, but it is not likely to sway critics of the realist credo. On the one hand, alternatives to the realist position are not very seriously considered. On the other hand, the relation between argument and evidence will not satisfy everyone.

Mearsheimer does not fully engage the rich body of theory that is critical of the realist perspective. Given the complexity of the democratic peace theory, and the extensive body of research on which it now rests, one could have wished for a more thorough attempt to wrestle with the conceptual and empirical claims made on its behalf. The role of international institutions, likewise, is dealt with in a rather peremptory manner, as is the constraining impact of international interdependence.

More importantly, Mearsheimer's empirical evidence may be questioned. The study focuses on Japan (from the Meiji Restoration to the defeat in World War II), Germany (from Bismarck until Hitler's defeat), the Soviet Union (from 1917 to 1991), Great Britain/the United Kingdom (from 1792 until 1945), and the United States (from 1880 to 1990). It also includes an examination of Italy (1861–1945), and a special focus on the U.S.-Soviet nuclear arms race. On the basis of these case studies, Mearsheimer finds that his theory of offensive realism is powerfully vindicated. But the evidence may be challenged. Some might consider the choice of cases a bit arbitrary, wondering how the thesis would have fared had France and China been included as case studies, or how it would have stood up to a post-1945 examination of Britain, Germany, and Japan. Even within the chosen case studies, not all historians would agree that the evidence demonstrates that these countries behaved as offensive realism would have predicted. For example, it is far from certain that the Soviet Union was intent on conquering Europe after World War II (as Mearsheimer maintains), or that British policy in the late nineteenth century amounted to a grab for offensive power.

The most serious problem, from an empirical point of view, is that it is not apparent what evidence, if any, could have disconfirmed Mearsheimer's thesis. The empirical test is not designed to allow for the possibility that a state that has not attained regional hegemony could appear content with the status quo. There is no meaningful variation in the circumstance to be predicted (a quest for offensive power), and we are faced with what more closely resembles a teleological than a scientific argument.

In any event, this study deals only with major powers. It is unclear whether the thesis is meant to apply to weaker nations, and one wonders how strong a case for offensive realism would emerge from a scrutiny of the policies of, say, Sweden, Hungary, Mexico, or Egypt. In as much as such countries, no less than major powers, are part of an anarchic international system, there is no reason why the argument should not encompass them, too. Yet careful examination probably would show that most countries fear others only under special circumstances: when there is an objective conflict of interest between them (or, perhaps, when one thinks the other is domestically impelled to act aggressively), not otherwise. This may apply to all countries, major powers included. If so, more would be gained by seeking to understand the circumstances causing nations to fear one another, rather than by taking as axiomatic the unproven assumption that they always and necessarily do.

This is an ambitious and well-written statement of an important theoretical position. As an empirical enterprise, it is what has been referred to as a “parallel demonstration of theory” with historical case studies: It illustrates a theory's potential fruitfulness by illustrating how the theory could have worked; it does not, however, show that it does work.