From the start of the post–Cold War era, analysts have debated whether US unipolarity would soon end and what sort of system would replace it. Early concerns that US dominance would quickly be counterbalanced soon gave way to expectations that unipolarity would long endure (Kenneth Waltz, “The Emerging Structure of International Politics,” International Security 18 (2), 1993; William Wohlforth, “The Stability of a Unipolar World,” International Security 24 (1), 1999). Starting in the 2000s, however, the effects of the United States’ domestic problems, China’s economic and military rise, and—for some—mounting Russian assertiveness in Europe moved the pendulum in the other direction. Although the jury is still out, a growing chorus concludes that unipolarity is waning, and as the 2017 National Security Strategy declares, a “great power competition” is at hand (for discussion, see Barry Posen, “From Unipolarity to Multipolarity: Transition in Sight?” in International Relations Theory and the Consequences of Unipolarity, eds. G. John Ikenberry, Michael Mastanduno, and William Wohlforth, 2011; John Mearsheimer, “The Gathering Storm: China’s Challenge to US Power in Asia,” Chinese Journal of International Politics 3 (4), 2010; Michael Beckley, Unrivaled: Why America Will Remain the World’s Sole Superpower, 2018; Donald Trump, “National Security Strategy of the United States of America,” 2017).

Making a significant contribution to this discussion is Øystein Tunsjø’s The Return of Bipolarity in World Politics. Tunsjø’s volume addresses two questions at the core of the debate: (1) what constitutes a pole in the international system, and how many poles are there, and (2) what sort of international behaviors follow from these different conditions. Working within a structural realist framework, Tunsjø proceeds in two parts. Drawing on the idea that states must have a combination of economic strength, military capabilities, geographic reach, political stability and competence, and self-sufficiency to qualify as poles, the volume first carefully assesses the contemporary distribution of power. Tunsjø’s conclusion—that the world is bipolar, with the United States and China vying for dominance—challenges not only claims of enduring unipolarity but also arguments that the world is shifting toward some kind of multipolarity.

This does not mean, however, that the system is headed for a “new Cold War” akin to that prevailing during the US-Soviet bipolar contest (see Odd Arne Westad, “Has a New Cold War Really Begun?” Foreign Affairs [Snapshot], 2018). Following Robert Jervis, Stephen Van Evera, and others, Tunsjø notes that structural realism’s expectation that similar distributions of power produce similar systemic behaviors is only partly right: power may explain broad trends in who and how states balance, but there is often variation in the intensity and scope of balancing efforts (canonical works here include Robert Jervis, “Cooperation under the Security Dilemma,” World Politics 30 (2), 1978; Thomas Christensen and Jack Snyder, “Chain Gangs and Passed Bucks: Predicting Alliance Patterns in Multipolarity,” International Organizations 44 (2), 1990; Stephen Van Evera, Causes of War: Power and the Roots of Conflict, 1999).

To explain this, Tunsjø attempts to incorporate “geopolitics”—especially the difference between maritime and continental domains—as a conditioning variable affecting calculations of strategic opportunities and interests. Because East Asia is a maritime arena in which deterrence at sea is difficult and spheres of influence opaque, the chance for miscalculation between the United States and China is greater than when the United States and the Soviet Union confronted one another in continental Europe. There is thus a greater risk of “limited war” even if, by Tunsjø’s logic, nuclear weapons and the “stopping power of water” may keep any such conflict from escalating (p. 130).

There is much to like in the project. The work stakes out an innovative position in the debate over the coming structure of international politics—one need not agree with the book’s findings to appreciate the importance of the argument. Meanwhile, the attempt to theorize the role of geopolitics expands on prevailing claims in the literature that certain geographic features impede great power competition (see, among others, Barry Posen, The Sources of Military Doctrine: France, Britain, and Germany between the World Wars, 1984; John Mearsheimer, The Tragedy of Great Power Politics, 2001; Robert Ross, “The Geography of the Peace: East Asia in the Twenty-First Century,” International Security 23 (4), 1999). Instead, Tunsjø compellingly argues that even defense-dominant geographic features do not necessarily hinder competition: they simply cause the form to morph. All of this is to the volume’s credit.

That said, no volume is perfect, and The Return of Bipolarity suffers from some ambiguities. This is especially the case with the discussion of geopolitics, where the work contains an underlying tension regarding what exactly geography does to the likelihood of great power conflict. On the one hand, Tunsjø claims that maritime domains limit the risk of military escalation amid a crisis or war: because boundaries are (by definition) fluid and the threat of conquest is limited, states’ incentives to push a fight to the maximum are much reduced relative to continental domains. This claim is broadly consistent with other work on geography and international conflict. On the other hand, the project claims that the opacity of spheres of influence, the unclear stakes involved, and the difficulty of maritime deterrence make limited war between the United States and China more likely than in continental domains. In short, maritime spaces simultaneously increase and decrease the conflict propensity of a given system.

These claims are each plausible, but they do not mesh well theoretically. Instead, the project would be stronger with greater discussion of the underlying reasoning and assumptions driving these propositions. As it stands, it seems an argument can be made in the opposite direction: maritime domains may limit the likelihood of even limited wars erupting because—to the volume’s point—spheres of influence are flexible, immediate military challenges are reduced, and states thus have time and room to negotiate rather than resort to force. If some kind of limited conflict between the United States and China is possible in East Asia, it may then not be due to regional geopolitics but rather because shifts in power tend to generate uncertainty and miscalculation (see Geoffrey Blainey, The Causes of War, 1988). The structural move toward bipolarity, in other words, may be more important in increasing the risk of war than the maritime nature of East Asian competition. Further suggesting the underlying theoretical problem, the inverse argument is also possible: if maritime domains make limited war possible, it requires significant faith in the conflict-impeding effects of water to also claim escalation is avoidable. Interestingly, in places (e.g., pp. 129–31, 148), Tunsjø seems to sense the latter tension and walks back claims surrounding escalation.

These issues aside, The Return of Bipolarity in World Politics is a major contribution to the onrushing literature on modern great power competition. Challenging prevailing claims surrounding the structure of world politics and the dynamics of bipolarity, Tunsjø’s volume requires careful engagement. As the shape of international relations changes as the twenty-first century wears on, analysts need more such projects that cut across regional and theoretical divides.