The United States emerged from the 1990s as an unrivaled global power to become a “unipolar” state. This extraordinary imbalance has triggered global debate. Governments and peoples around the world are struggling to understand to how an American-centered unipolar system operates—and to respond to it. What is the character of domination in a unipolar distribution? To what extent can a unipolar state translate its formidable capabilities into meaningful influence? Will a unipolar world be built around rules and institutions or be based more on the unilateral exercise of unipolar power? Scholars too are asking these basic questions about unipolarity and international relations theory. The individual contributions develop hypotheses and explore the impact of unipolarity on the behavior of the dominant state, on the reactions of other states, and on the properties of the international system. Collectively, they find that unipolarity does have a profound impact on international politics. international relations under conditions of unipolarity force a rethinking of conventional and received understandings about the operation of the balance of power, the meaning of alliance partnerships, the logic of international economic cooperation, the relationship between power and legitimacy, and the behavior of satisfied and revisionist states.

Most scholars hold that the consequences of unipolarity for great power conflict are indeterminate and that a power shift resulting in a return to bipolarity or multipolarity will not raise the specter of great power war. This article calls into question the core assumptions underlying the consensus: (1) that people are mainly motivated by the instrumental pursuit of tangible ends such as physical security and material prosperity and (2) that major powers' satisfaction with the status quo is relatively independent of the distribution of capabilities. in fact, it is known that people are motivated powerfully by a noninstrumental concern for relative status, and there is strong empirical evidence linking the salience of those concerns to distributions of resources. If the status of states depends in some measure on their relative capabilities and if states derive utility from status, then different distributions of capabilities may affect levels of satisfaction, just as different income distributions may affect levels of status competition in domestic settings. Building on research in psychology and sociology, the author argues that even capabilities distributions among major powers foster ambiguous status hierarchies, which generate more dissatisfaction and clashes over the status quo. and the more stratified the distribution of capabilities, the less likely such status competition is. Unipolarity thus augurs for great power peace, and a shift back to bipolarity or multipolarity raises the probability of war even among great powers with little material cause to fight.

Despite preponderant power, unipoles often do not get their way. Why? Scholars interested in polarity and the systemic structures determined by the distribution of power have largely focused on material power alone, but the structure of world politics is as much social as it is material. In this article the author explores three social mechanisms that limit unipolar power and shape its possible uses. The first involves legitimation. To exercise power effectively, unipoles must legitimate it and in the act of legitimating their power, it must be diffused since legitimation lies in the hands of others. The second involves institutionalization. A common way to legitimate power is to institutionalize it. Institutionalizing power in rational-legal authorities fundamentally transforms it, however. Once in place, institutions, laws, and rules have powers and internal logics of their own that unipoles find difficult to control. The third relates to hypocrisy. The social structures of legitimation and insti tutionalization do more than simply diffuse power away from the unipole; they create incentives for hypocrisy. Hypocrisy is a double-edged sword for unipoles. On the one hand, unrestrained hypocrisy by unipoles undermines the legitimacy of their power. On the other hand, judicious hypocrisy can provide crucial strategies for melding ideals and interests. Indeed, honoring social ideals or principles in the breach can have long-lasting political effects, as decades of U.S. hypocrisy about democratization and human rights suggest.

Unipolarity is a novel condition in world politics, and its effects on international alliances have yet to receive sustained theoretical attention. Tracing its impact requires a careful distinction between the purely structural features common to any unipolar system and the unique characteristics of the current unipole (the United States) or the policies undertaken by particular U.S. leaders (such as George W. Bush). In general, the unipole will enjoy greater freedom of action and be less dependent on allied support, enabling it to rely more readily on ad hoc “coalitions of the willing.” Lesser powers will be concerned about the concentration of power held by the unipole, but they will also face larger barriers to concerted action to contain it. Hard balancing against the unipole will be unlikely—unless the unipole begins a major effort to expand—but lesser powers will engage in soft balancing to contain the latter's influence. Medium powers may pursue alliances with others in order to reduce dependence on the unipole, but weaker states are likely to ally with the unipole in order to use its power against local security challenges. Bandwagoning will remain rare even under unipolarity, but disputes over burden sharing and alliance leadership will continue. Weaker states will prefer multilateral arrangements that enhance their own influence, while the unipole will prefer bilateral or ad hoc coalitions of the willing that it can more readily dominate.

There is striking consistency in the international economic behavior of the United States across the bipolar and unipolar eras. The United States has been simultaneously a system maker and privilege taker, and its ability to play that dual role has required the willing collaboration of foreign partners. U.S. influence over those partners, however, has changed in important ways. During the cold war the United States dominated international economic adjustment struggles. Its ability to prevail in those struggles after the cold war has been significantly compromised. The United States, notwithstanding its preponderant power, no longer enjoys the same type of security leverage it once possessed, and the very success of the U.S.-centered world economy has opened a greater range of international and domestic economic options for America's supporters. In the unipolar era the United States may continue to act its own way, but it can no longer count on getting its own way.

Under unipolarity, the immediate costs and risks of war are more likely to seem manageable for a militarily dominant power like the U.S. This does not necessarily make the use of force cheap or wise, but it means that the costs and risks attendant on its use are comparatively indirect, long term, and thus highly subject to interpretation. Unipolarity, combined with the opportunity created by September 11, opened a space for interpretation that tempted a highly ideological foreign policy cohort to seize on international terrorism as an issue to transform the balance of power both in the international system and in American party politics. This cohort's response to the terrorist attack was grounded in ideological sincerity but also in the routine practice of wedge issue politics, which had been honed on domestic issues during three decades of partisan ideological polarization and then extended into foreign policy.

In analyzing the current unipolar system, it is useful to begin with structure. No other state or plausible coalition can challenge the unipole's core security, but this does not mean that all its values are safe or that it can get everything that it wants. Contrary to what is often claimed, standard balance of power arguments do not imply that a coalition will form to challenge the unipole. Realism also indicates that rather than seeking to maintain the system, the unipole may seek further expansion. To understand the current system requires combining structural analysis with an appreciation of the particular characteristics of the current era, the United States, and its leaders. Doing so shows further incentives to change the system and highlights the role of nuclear proliferation in modifying existing arrangements.