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World Out of Balance:
International Relations and the Challenge of American Primacy
Stephen G. Brooks & William C. Wohlforth

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THE DISSOLUTION OF THE SOVIET UNION marked the emergence of historically unprecedented U.S. advantages in the scales of world power. No system of sovereign states has ever contained one state with comparable material preponderance.1 Following its invasion of Afghanistan in 2001, the United States loomed so large on the world stage that many scholars called it an empire,2 but the costly turmoil that engulfed Iraq following the toppling of Saddam Hussein in 2003 quieted such talk. Suddenly, the limits of U.S. power became the new preoccupation. Many analysts began to compare the United States to Britain at the beginning of the twentieth century—an overstretched, declining, “weary Titan” that “staggers under the too vast orb of his fate.”3

What accounts for this sudden shift in assessments of American power? For most observers, it was not new information about material capabilities. As Robert Jervis observes, “Measured in any conceivable way, the United States has a greater share of world power than any other country in history.”4 That statement was as accurate when it was written in 2006 as it would have been at any time after 1991, and the primacy it describes will long persist, even if the most pessimistic prognostications about U.S. economic, military, and technological competitiveness come true. For most scholars of international relations, what really changed after 2003 were estimates of the political utility of America’s primacy. Suddenly, scholars were impressed by the fact that material preponderance does not always translate into desired outcomes. For many, theories of international relations (IR) that explain constraints on the use of power were vindicated by American setbacks in Iraq and elsewhere.

For more than three decades, much IR scholarship has been devoted to theories about how the international environment shapes states’ behavior.5 Applying them to the case at hand, scholars have drawn on each of the main IR theories—realism, institutionalism, constructivism, and liberalism—to identify external (or “systemic”) constraints that undermine the value of the United States’ primacy, greatly restricting the range of security policies it can pursue. Scholars emphasize a variety of elements in the international system that constrain U.S. security policy: international institutions, balancing dynamics, global economic interdependence, and legitimacy. The upshot is simple but portentous for the contours of international politics in the decades to come: the political utility of U.S. material primacy is attenuated or even negated by enduring properties of the international system.

The purpose of this book is to undertake a systematic evaluation of the external constraints that scholars have highlighted and thereby gain a better understanding of the United States’ global role. This entails answering four questions: Does the United States face the imminent prospect of having its power checked by a balancing coalition of other great powers? As it has become increasingly exposed to the international economy, has the United States become more vulnerable to other actors’ attempts to influence its security policies? Is the United States tightly bound by the need to maintain a good general reputation for cooperation in international institutions? Does the United States need to adhere to existing rules to sustain legitimacy and thus maintain today’s international institutional order?

Our answer to each of these questions is no—a finding that overturns the scholarly conventional wisdom, according to which these factors strongly constrain U.S. security policy. On the contrary, the unprecedented concentration of power resources in the United States generally renders inoperative the constraining effects of the systemic properties long central to research in international relations.

Given the likely longevity of American primacy, this general finding has important repercussions for thinking about international relations scholarship and U.S. foreign policy. In the concluding chapter, we outline a new research agenda to address the analytical challenge of American primacy, and identify an important and heretofore neglected grand strategic alternative for the United States.


Our purpose is to analyze propositions drawn from all the theoretical schools that deal with the systemic constraints on U.S. security policy. Following many other scholars, we treat security policy as not simply the use and threat of military force, but also the use of nonmilitary tools to advance security interests. By systemic constraints, we mean constraints that are external to the United States itself, and that operate in the international system generally rather than within one set of actors or in response to a particular issue. More specifically, a systemic constraint is a property of the international system that restricts freedom of action by forbidding, or raising the costs of, certain kinds of actions, or compelling other kinds of actions.

Scholars stress that the shift from the bipolarity of the Cold War to the current unipolarity is not an unalloyed benefit for the United States because it comes with the prospect of counterbalancing, increased dependence on the international economy, a greater need to maintain a favorable reputation to sustain cooperation within international institutions, and greater challenges to American legitimacy. The conventional wisdom is that these systemic constraints impede the translation of U.S. power capabilities into influence over security outcomes, rendering the United States much less capable than its material capabilities imply. Put more generally, existing theoretical arguments sum up to the contention that once a state is at or near the top of the international heap, it confronts more and stronger properties of the international system that greatly diminish the marginal utility of additional capabilities for pursuing its security objectives.6

The validity of this view depends on whether systemic constraints function in a unipolar system as they did in the bi-and multipolar systems on which most IR research is based. Yet answering that critical question has not been the explicit object of study. As a result, the research underlying the conventional wisdom suffers from one or more of the following problems: it uncritically applies theories developed to explain past international systems; it does not subject arguments to systematic theoretical or empirical analysis; it considers only a single theoretical perspective; and it is not specifically focused upon the constraints on U.S. security policy. To assess the conventional wisdom, it is necessary to examine the key systemic constraints to determine whether and to what degree their operation is transformed in a unipolar system. Ours is the first book to do this, and it does so for all the systemic constraints highlighted by IR theory.

This study turns the conventional wisdom on its head: our assessment is that as the concentration of power in a state increases beyond a certain threshold, systemic constraints on its security policy become generally inoperative. Scholars are right to hold that systemic constraints are potentially important, but wrong to assume that theories developed to explain previous international systems apply to unipolarity.


Two sets of constraints on U.S. security policy are featured in the scholarly literature: systemic constraints and those that emanate from the United States’ domestic politics and institutions. The core domestic question is whether the public acts as a constraint on American security policy.7 Most of the scholarship focuses on how the public reacts to the use of force, and finds that the effect of public opinion varies according to case-specific factors, including the perceived likelihood of battlefield success,8 the number of actual or expected casualties,9 the nature of discourse among policymakers and political elites,10 the objectives in a given case,11 whether the mission is backed by multilateral institutions,12 and the nature of media coverage.13 The significance of public support or opposition also depends on the normative and political beliefs of the particular president who is fashioning policy.14

In contrast to these complex influences is scholars’ stark portrait of the systemic constraints facing the United States: rising power meets rising constraints. Perhaps because of the appeal of this relative clarity, scholars who evaluate U.S. policy generally focus on systemic constraints.15 Their conclusions, however, are not backed up by research that is as careful as that which addresses domestic constraints. Instead, their stark perspective on systemic constraints is initially plausible because it resonates with decades of theorizing on international relations.

Since World War II scholars have pursued general, systematic knowledge about international relations. Starting in the 1950s, this led to a preoccupation with systemic theory. A hallmark of the approach is its commitment to general explanations of patterns over long spans of time, as opposed to details of specific interstate interactions. Scholars developed and tested general propositions about the social system of states with little reference to their internal properties.16 Even though the influence of systemic theory declined in the late 1980s, most research in the field still either reacts to or develops this approach.17 Today’s scholarship concerning constraints on the United States is the product of this intellectual history.

The provenance of the conventional wisdom on systemic constraints is clearest for realism. Indelibly associated with realism is balance-ofpower theory, a quintessential theory of systemic constraints. It stipulates that the absence of a central authority that can enforce agreements (i.e., the condition of anarchy) puts a premium on states’ long-term survival (security), which leads them to counter potentially dangerous concentrations of power (which balance-of-power theorists frequently call hegemony) through alliances (external balancing) or military buildups (internal balancing). According to the theory, the stronger a state gets, the more powerful become the incentives for other states to balance it. “Hegemony leads to balance,” Kenneth N. Waltz observes, “through all of the centuries we can contemplate.”18

It is little wonder that scholars reached for this theory to analyze systemic constraints on the United States after the Cold War. No other single proposition about international politics has attracted more scholarly effort than the balance of power. It is perhaps as central in today’s thinking as it has been at any time since the Enlightenment, when Rousseau and Hume transformed familiar lore about balancing diplomacy into coherent theoretical arguments.19 Waltz, who turned those arguments into a structural systemic theory in the 1970s, has been one of the most influential scholars of international relations over the last three decades. The theory’s basic proposition, the self-negating nature of power, seemed tailor-made for the post–Cold War era, when the United States assumed unprecedented material preponderance in the international system.

Remarkably, scholars from normally competing theoretical traditions have reached similar conclusions about the self-negating nature of contemporary American power. Institutionalist, constructivist, and modern liberal theories all developed in part as critical reactions to realism. All reject simple power-centric models like the balance of power; all feature causal mechanisms that are downplayed or ignored in realist writings. Yet these theoretical schools reach the same general conclusion about the constraints facing the United States today: as its share of power in the international system increases, the systemic constraints on U.S. security policy also increase (though the link between them is not a matter of balancing—the causal pathways are less direct and linear than realism’s notion of power begetting countervailing power).

Institutionalist theory shows how states gain from cooperating within international institutions and, conversely, how much they can lose if they fail to cooperate in a world with high levels of interdependence. To avoid these losses, institutionalists stress, states must bind themselves to institutional rules. While these constraints apply to all states that want to benefit from institutionalized cooperation, they are, according to recent analyses, especially salient for the leading state. As it becomes more powerful—as when the relative power of the United States increased with the Soviet Union’s fall—it has a greater ability to exempt itself from inconvenient institutional rules of the game without being punished in the short term.20 Therefore, “the more that a powerful state is capable of dominating or abandoning weaker states, the more the weaker states will care about constraints on the leading state’s policy autonomy.”21

The basic proposition emerging from institutionalist scholarship is that the United States faces a critical need to maintain a favorable reputation for international cooperation; any effort to revise or insulate itself from the current institutional order is dangerous, institutionalists maintain, because it will undermine America’s “multilateral reputation,” reducing other states’ cooperation in areas where Washington strongly values it, such as trade. If true, this argument has major implications for U.S. security policy: to sustain institutionalized cooperation from weaker countries, the United States more than other nations needs to accept the constraints associated with multilateral agreements and rules.

Constructivist scholarship makes a similar argument regarding the constraining force of the international order, in which the concept of legitimacy plays the key role. Constructivists emphasize that America’s material resources can translate into political influence only when they are bound by the rules of the institutional order. Christian Reus-Smit summarizes the core claims, namely “that all political power is deeply embedded in webs of social exchange and mutual constitution; that stable political power . . . ultimately rests on legitimacy; and that institutions play a crucial role in sustaining such power.”22 It follows that the more powerful a state is, the more it has to gain by legitimizing its power, and the more it has to lose if others question that legitimacy. The shift from bi-to unipolarity has magnified the salience of this basic proposition. Constructivist scholarship thus generates an argument with profound implications for the United States: failure to hew to the accepted rules in the security realm will degrade American legitimacy and thereby complicate and weaken American hegemony.23 However inconvenient accepted practices may be, departing from them will erode the foundations of American hegemony.

Thus, both constructivist and institutionalist analyses emphasize the institutional constraints on U.S. security policy. Liberalism, meanwhile, points to another aspect of the international environment: global economic interdependence, which has accelerated dramatically in recent decades. The liberal proposition is that economic interdependence can constrain the security policies of states, including those at the top of the power hierarchy.24 This effect is particularly significant today because the opportunity cost of reduced access to the world economy is now so high. By embracing globalization to an ever greater extent, the United States has enhanced its economic capacity and hence its overall power. However, this is a Faustian bargain, according to many analysts, because U.S. security policy is more exposed to potential constraints associated with economic interdependence.25

The convergence of all of the major schools in international relations on the same basic argument regarding systemic constraints is grounded in scholarship: in each case, the proposition that rising power generates rising constraints is a reasonable first-cut inference from existing theoretical and empirical analyses. It is hardly surprising that when confronted with the novel condition of unipolarity, scholars plumbed existing theories and research for inferences regarding constraints on the United States. After all, scholars are only human. They are cognitive misers. When the world changes, they do not abandon all their theories and start afresh. “Instead of radical change,” Jack Snyder notes, “academia has adjusted existing theories to meet new realities.”26

While this reaction is understandable, it is not optimal in the long run. The degree of U.S. dominance is unprecedented, and this alone is enough to place a question mark after inferences derived from research on previous systems. Unfortunately, the incentive to subject conventional wisdom to theoretical and empirical scrutiny is reduced when normally competitive theoretical schools converge on a proposition— one that initially seems compelling and also aligns with the prescription for foreign policy the overwhelming majority of IR scholars would endorse: restraint in the face of the temptations of power.

However, given the importance of systemic constraints on power for evaluating both U.S. foreign policy and international security more generally, scholarly assessments should rest on firm foundations. Our book provides a thorough analysis of the five key theoretical arguments concerning the systemic constraints facing the United States today.


The question for us is this: how do systemic constraints featured in IR scholarship operate on the United States, given its weight in the interstate scales of power? To address this question, we need to be clear about what we mean by power and the strength of various constraints.

Power and Polarity

In the wake of a Cold War that had been shaped by two leading states, analysts recognized that a world without a nation capable of rivaling the United States would be different in important ways. Observers grappling with the post-bipolar international system have characterized it in such terms as empire, unipolarity, imperium, and uni-multipolarity.27 These terms reflect a search for theoretical constructions to place in historical and comparative perspective the distinctive political formation that has taken shape around American power. But our analysis concerns constraints on the conversion of material resources into desired outcomes. That topic requires a basic distinction between power as material resources and power as the ability to realize ends.28 Following the practice of many scholars, we use the term power in the former sense to denote the resources a government can draw upon. The global system today—seen in comparative historical perspective— has very concentrated means of power. Using the term power to denote these material capabilities does not prejudge the character of influence or the logic of political relationships within the global system.29

Our analysis does not hinge on the particular term used to describe the current system; our analytical starting point—that the United States has a greater share of power than any single state has ever had in 300 years—is uncontested among IR scholars. Of all the shorthand terms to describe the current system, unipolarity is the most accurate and presents the smallest risk of conflating power as resources with power as political relations of influence and control. Some discussion of the applicability of this term helps to put the unique nature of today’s system in sharper relief.

Scholars use the term unipolarity to distinguish a system with one extremely capable state from systems with two or more such states (bi-, tri-, and multipolarity), and from empire, which generally refers to relations of political influence and control rather than distributions of capability. The adjective unipolar describes something that has a single pole. To occupy a pole in the international system, a state must (a) command an especially large share of the resources states can use to achieve their ends; and (b) excel in all the component elements of state capability (conventionally defined as size of population and territory, resource endowment, economic capacity, military might, and organizational-institutional “competence”).30 By definition, in a unipolar system only one state meets these criteria.

The concept of polarity has deep roots in scholarship on international relations. The core contention is that polarity structures states’ probable actions, providing incentives and disincentives for different types of behavior. However, the concept yields few important insights into patterns in international politics over the long term. Even those scholars most persuaded of its analytical utility see polarity as a necessary component of, rather than a complete, explanation of behavior.31 In part because it suggests a dependence on Kenneth Waltz’s writings on polarity, the term is not ideal for our purposes. As we make clear in the chapters that follow, our analysis is not based on the neorealist system of explanation; because we seek to evaluate each school of thought on its own terms, our approach is theoretically agnostic. That said, the concept of polarity is an efficient way to keep clear the vital distinction between power as resources and power as the ability to attain desired ends.

Polarity is a theoretical construct; real international systems only approximate ideal types. The concept of unipolarity implies a threshold value in the distribution of capabilities among states. How do we know whether a system has passed the threshold, becoming unipolar? According to the definition of a pole presented earlier, an international system is unipolar if it contains one state whose share of capabilities places it in a class by itself compared to all other states. This definition reflects the fact that a state’s capabilities are measured not on an absolute scale but relative to those of other states. In keeping with this definition, a unipolar state is preponderant in all relevant categories of capability.32 According to a narrower, but also frequently used, criterion, a system is unipolar if it has only one state capable of organizing major politico-military action anywhere in the system.33

There are periods of history about which scholars disagree over polarity, but ours is not one of them. By consensus, four or more states qualified as poles before 1945; by 1950 or so, only two measured up; and now one of these poles is gone. Since the dissolution of the Soviet Union, no other power—not Japan, China, India, or Russia, nor any European country, nor the European Union—has increased its capabilities sufficiently to transform itself into a pole. The United States alone retains the wherewithal to organize major politico-military action anywhere in the system. The more definite is American material preeminence, the more apt is the term unipolarity. As the empirical analysis in chapter 2 shows, in today’s system the term is very apt indeed.

The Power of Constraints

Clarity about our definition of power must be accompanied by clarity about constraints. Each of the theoretical arguments we shall assess is complex, often subsuming numerous propositions, and each has been articulated in subtle ways by different scholars. Although these theories sum up to what we have called a conventional wisdom on systemic constraints, scholars make distinct claims about the strength of the constraints their theories identify.34

Constraints vary along two key dimensions. The first is conditionality. The key issue is whether the constraint is triggered by a state’s policies: as we move along this continuum from the less to the more conditional, a constraint may be inescapable, avoidable if a state pursues appropriate policies, or inapplicable no matter what it does. The second dimension is strength. A strong constraint is one that significantly reduces the practical utility of a state’s power resources, whereas a weak constraint has a less consequential effect. The strength of a constraint is a function of both its scope—that is, how large a range of policies it applies to—and how malleable and reversible it is.

These considerations produce a continuum of constraints on U.S. security policy as follows:

  1. A structural constraint exists as a result of the United States’ position in the international system; it will affect any effort to use power in the pursuit of security, no matter what policies the United States follows. Theoretically, a structural constraint may be weak—that is, it only marginally affects the utility of resources—but in practice, scholars almost always contend that structural constraints are strong. As a result, it is not necessary to distinguish between weak and strong structural constraints, since the former do not exist within IR scholarship.
  2. A strong conditional constraint powerfully affects the ability to use resources in pursuit of security interests, but is triggered only if the United States adopts certain policies.
  3. A weak conditional constraint imposes relatively minor impediments on the use of power to advance security interests, and then only if the United States adopts certain policies.
  4. A constraint is inoperative if it is extremely unlikely to apply to the United States under unipolarity, or its scope and significance is so restricted as to render it essentially inconsequential.

The conventional wisdom in IR scholarship is that the international environment tightly constrains U.S. security policy: most systemic constraints that scholars highlight are strongly conditional, and some are structural. Our conclusion, in contrast, is that the international environment does not tightly constrain U.S. security policy; systemic constraints are generally inoperative.


The core of this book is a thorough evaluation of theoretical arguments, but the results of this assessment directly bear on three overarching questions of policy: Can the United States sustain an expansive range of security commitments around the globe? Is the United States well positioned to reshape the international system to better advance its security interests? What are the general costs of unilateralism?

A contentious debate is under way over how large a security “footprint” the United States should have. Three positions dominate this debate: “offshore balancing” (the United States should sharply reduce its security commitments and military deployments overseas, pulling back toward its own borders);35 “engagement” (the United States should maintain the security role and military profile it had overseas prior to 9/11, wary of any significant expansion);36 and “primacy” (the United States should augment the footprint it had overseas before 9/11).37

Our general conclusion, that the United States does not face strong systemic constraints, has great relevance for this debate. Specifically, if current IR scholarship is right, then, because of external constraints, the United States will have difficulty maintaining its current security profile (engagement) let alone enhancing its military footprint (primacy). It is perhaps not a coincidence that many who advocate the remaining option (offshore balancing) have also argued that the United States now faces very strong systemic constraints on its security policy.38

Our analysis, by contrast, shows that the systemic environment does not undermine, let alone rule out, any of the three options. But it does not show what choice the United States should make. It is important to distinguish dispassionate analysis of the underlying structure of international politics from advocacy for one strategic choice.39 We argue from theory and evidence that the current unipolar system is durable and that the systemic constraints on U.S. security policy are generally inoperative. One can agree with our assessment of the systemic environment while promoting any of the three grand strategies reviewed above, including offshore balancing.

The debate about the long-term direction of U.S. security policy is often restricted to how and where the United States deploys its military resources. Largely unaddressed is a second issue, one for which our analysis has important implications: whether the United States should consider changing the international system. In his groundbreaking book War and Change in World Politics, Robert Gilpin argued that leading states “will attempt to change the international system if the expected benefits exceed the expected costs.”40 In the quarter century since that book’s publication, scholars have never seriously debated whether the “expected net gain” of such change might be positive for the United States. It is hardly surprising that scholars set aside the question of large-scale revisions of the territorial status quo—plausible arguments for the utility of widespread conquest in an age of nuclear weapons and low economic benefits of holding territory are hard to imagine. But Gilpin emphasizes that revising the territorial status quo is only one of three objectives that powerful states might pursue; the other two are nonterritorial: gaining influence over the global economy, and “creating an international political environment and rules of the system that will be conducive to their political, economic, and ideological interests.”41 Why is there no sustained scholarly debate on the costs and benefits of system change in pursuit of these nonterritorial objectives?

The answer is in assessments of the distribution of power. In the 1980s, scholars believed that the United States was in relative decline. The costs of changing the system would thus be too high, and conservatism was the order of the day. With the collapse of Soviet power in 1989–91 came a dramatic shift of power in favor of the United States, presumably increasing the attractiveness of system change. Yet most observers assumed that unipolarity was but a “moment,” and so long-range projects of systemic activism did not appear germane.

By the end of the millennium, however, most scholars accepted that unipolarity was not about to erode any time soon, and still the question of U.S. systemic activism was neglected. This inattention can be traced to two prevalent assumptions. The first is that any effort to revise the system would be fruitless, costly, or both, in large part because of systemic constraints on the exercise of power. John Ikenberry, for example, stresses the need for the United States “to operate through mutually agreed rules”42 and emphasizes that “the more willing the U.S. is to act within institutional constraints and tie itself to others . . . the less likely it is that states will seek to balance against it or seek to establish a rival international order.”43 The second assumption is that, in the words of Robert Jervis, “[t]he current international system, although not necessarily perfect, is certainly satisfactory.”44 These assumptions yield a negative cost-benefit ratio for U.S. efforts to revise the system even if unipolarity will long endure. And if activism makes no sense, then conservatism is the only practical route. This perhaps explains why IR scholars have been so reluctant to address the question of system change, and why they instead counsel the United States to be a “very conservative state” and to “seek to maintain the prevailing international system.”45

This book reveals that the first assumption underlying conservatism has no basis. As we show, systemic constraints on U.S. security policy do not rise with American power; there is no reason to expect that for the next two decades external constraints will meaningfully impede U.S. efforts to revise the international system. It then becomes clear that the second assumption at the root of conservatism is debatable, and ought to be debated. Our concluding chapter addresses the pressing need to begin a serious discussion of the potential security benefits of revising the system. That debate would both provide helpful guidance to policymakers and lead to a better understanding of the true security environment in today’s unipolar system and how U.S. policymakers are likely to respond to it.

The final long-term policy issue our analysis bears upon is the costs of unilateralism.46 IR scholars invariably see going it alone as costly, particularly for the United States today. Stanley Hoffman’s warning, that “nothing is more dangerous for a ‘hyperpower’ than the temptation of unilateralism,” is typical of scholarly assessments.47 The general argument is that unilateralism is prohibitively costly because it augments systemic constraints: enhanced efforts to balance U.S. power, reduced legitimacy of the U.S.-led international order, and a damaged American reputation that will curtail prospects for cooperation in international institutions. Our finding, that for the United States systemic constraints are generally inoperative, thus undermines the scholarly consensus on the high costs of unilateral policies.

This does not mean that unilateralism is wise. Any policy may be wise or unwise, and many unilateral policies pursued by the United States undoubtedly fall into the latter category. The core question is whether punishing general costs arise from unilateral policies regardless of their substance. The findings in this book provide no evidence for such costs, although scholars habitually write about them as if there were such evidence. Again, this result does not mean that the United States should be more or less unilateral, or more or less multilateral. What our findings reveal is that the benefit of acting multilaterally rests on the substance of a given policy, not on the purported general costs of unilateralism. Analysts must distinguish procedural criticisms of unilateral policies from criticisms based on substance. The benefits of acting unilaterally in particular circumstances need to be considered, not neglected because of the general presumption that systemic incentives ineluctably make such action costly and impractical.

A single point lies at the root of all three of these implications for policy: inoperative systemic constraints mean that, much more than scholars generally believe, U.S. foreign policy is a realm of choice, rather than necessity. IR scholars, now noticeably silent on what choices the United States should make on all three issues, must be heard. As we stress in our final chapter, the fact that IR scholarship currently cannot provide much guidance on optimal choices does not mean that it will never do so; rather, much more research on these issues is needed.


Our study is as wide-ranging as the theories we consider. It contains purely theoretical critiques, contemporary and historical case studies, and careful analysis of numerical data. Along the way, we develop and evaluate our own theoretical arguments about how balancing, globalization, legitimacy, and institutionalized cooperation operate in a unipolar world.

In chapters 2 and 3, we address balancing—the most prominent proposition within realism and, arguably, IR theory generally. The balancing proposition has two branches, balance-of-power theory and balance-of-threat theory. Chapter 2 addresses the former, which predicts that states will try to prevent the rise of a hegemon. While scholars debate the empirical veracity of this proposition historically, they have not registered a more important point concerning its implications for constraints on U.S. power today: Even if a potential hegemon needs to be concerned about a counterbalancing constraint, as the theory predicts, the theory does not yield this implication for a hegemon that is already firmly established. Indeed, we argue that once a country passes that threshold, the theory’s causal arrows are reversed.

Chapter 3 evaluates the argument on constraints that has been derived from balance-of-threat theory. Scholars who have applied the theory argue that balancing dynamics under unipolarity will, at least initially, operate more subtly than the counterbalancing predicted by balance-of-power theory. Arguably the most frequently cited systemic constraint in discussions of American foreign policy, this argument has never been tested empirically against alternative explanations. We present such a test, and find that the actions analysts regard as balancing are, in fact, better explained by factors that fall outside balance-of-threat theory.

The key liberal argument that enhanced economic interdependence leads to increased exposure to constraints on the security policy of the United States is the subject of chapter 4. The argument comes in many forms, but we show that most founder on the problem of “asymmetric interdependence”: the immense presence of the United States within the global economy makes other states more economically dependent on it than it is dependent on them. Precisely because it occupies such a dominant position, the United States is able to grow economically via globalization without the prospect that other countries will use economic statecraft to constrain its security policy. Thus, a systemic constraint widely thought to be strongly conditional—to use one of the terms we have defined above—is largely inoperative. Chapter 4 also evaluates more general, indirect mechanisms by which economic interdependence might constrain U.S. security policy. We find that rising economic interdependence is likely neither to change other countries’ ability or preference to constrain U.S. policies, nor lead to constraining actions by nonstate actors.

Chapter 5 addresses the institutionalist argument that the institutional order is imperiled if the United States does not strongly invest in maintaining a reputation for multilateralism. This core argument, we show, depends upon an assumption about the way reputations work that is theoretically implausible and empirically unsubstantiated. There is an alternative conception of reputations that rests on firmer theoretical foundations and is consistent with the empirical record. We thus find that the reputational constraint is inoperative, not strongly conditional as now posited by institutionalist scholarship.

Chapter 6 analyzes the key argument on constraints derived from constructivism, which concerns legitimacy. We establish several considerations that undermine the constructivists’ argument. Reviewing key episodes alleged to have imposed legitimacy costs on the United States, we demonstrate the contingent and malleable nature of the supposed constraint. Legitimacy both limits and enables power, and power can fuel legitimacy. The United States needs legitimacy, but the constraint this need imposes on U.S. security policy is conditional and weak, rather than conditional and strong or even structural, as constructivist treatments contend.

The concluding chapter discusses two chief implications. Rather than vindicating existing theories, the reversals and challenges U.S. foreign policy encountered after 2003 underscore the need for a new research agenda for IR scholars. To explain the nature of the constraints on U.S. security policy, scholars must shift away from the standard focus on the external environment and examine other kinds of factors. Analysts also need to examine how the United States can best take advantage of its unprecedented opportunity to change the international system in its long-term security interests. The debate about U.S. grand strategy needs to consider a new alternative: using American leverage to reshape international institutions, standards of legitimacy and economic globalization.

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