In this book, Nuno P. Monteiro points out that the field of international politics has no coherent theory of unipolarity, which he defines in military terms as an anarchical interstate system with a sole great power. The great structural theorist Kenneth N. Waltz regarded enduring unipolarity as unlikely since balancing will occur as “secondary states flock to the weaker side” (Theory of International Politics, 1979, 126). Two decades later, William Wohlforth argued that unipolarity should be associated with peace (“The Stability of a Unipolar World,” International Security 24 (no. 1 1999); 5–41). Indeed, for many theorists, following Thucydides, it is the rise of other great powers to challenge the hegemon that generates threats of war.

The world has now been unipolar for a quarter century, however, and U.S. military dominance seems secure; yet the United States has been involved in wars in Afghanistan and Kosovo and two wars in Iraq. The persistence of unipolarity and its apparently conflictual tendencies present anomalies for previous theories. Monteiro seeks to resolve these anomalies by arguing that “although unipolarity is potentially durable, it is likely to make room for significant levels of persistent conflict regardless of the grand strategy the United States chooses to implement” (p. 12).

Although Monteiro takes issue with Waltz on unipolarity, and adopts an assumption of state rationality that Waltz eschewed, Theory of Unipolar Politics is a Waltzian book. Even the title was deliberately chosen to parallel Theory of International Politics. Like Waltz, Monteiro has a self-conscious and sophisticated view of the philosophy of science, and, like Waltz, he deliberately ignores how domestic politics or ideas prompt states to take particular actions and downplays the role of multilateral institutions. He also seeks to fashion a parsimonious theory, focusing on incentives. Now that Waltz has passed away, Monteiro seems to be claiming his mantle as the most self-consciously structural-realist theorist in the discipline.

Addressing major theoretical problems has become unfashionable in a discipline currently obsessed with causal identification, often at the expense of studying big and important issues. But understanding unipolarity is essential if we are to understand the role of the United States in twenty-first-century world politics.

Under what conditions is unipolarity durable? In a non-nuclear world, unipolarity threatens other states, but in a nuclear world, assured second-strike capability ensures their survival. Hence, in a nuclear unipolar world, structure does not dictate balancing, and other states’ behavior will depend on the unipole’s strategy, which Monteiro distinguishes along two dimensions, military and economic. Militarily, unipoles can pursue offensive dominance, defensive dominance, or disengagement; economically, they can accommodate the growth of other economies or seek to contain it. The author argues that in a nuclear world, unipolarity is durable when the unipolar state pursues a strategy of “defensive accommodation,” seeking to maintain its military dominance but accommodating demands from other states for economic growth. Acting as a unipole, the United States has pursued defensive accommodation except in Europe during the 1990s and the Middle East between 2001 and 2008, when it pursued a strategy of offensive accommodation.

Chinese strategy in the early twenty-first century provides the key test of unipolar durability. If Monteiro is right, China should not challenge U.S. global dominance, focusing instead on maintaining a nuclear second-strike capacity and attaining such limited objectives as preventing permanent sovereignty for Taiwan. Consistent with his theory, China has developed only a small, secure, second-strike nuclear capacity and has refrained from building multiple-warhead missiles. Monteiro predicts that “China will not engage in a militarized challenge to the global status quo and will therefore allow the United States to remain in its unipolar position” (p. 143).

Monteiro does not, however, offer a strong rebuttal to those pessimists who fear that when China has attained greater economic power, it will mount a major challenge to U.S. dominance. He is remarkably sanguine about Chinese actions in the South China and East China Seas. Chinese actions during 2014, including its declaration of a no-fly zone outside its accepted territorial jurisdiction, cast doubt on his benign interpretation.

All is not truly well in the unipolar world since the potential for conflict is high. Indeed, Moneiro calculates that for the period between 1816 and 2014, the incidence of great-power war per great-power year has been about four times as high under unipolarity than under multipolarity or bipolarity. That is, although the lethality of war has fallen sharply, as others have argued, its incidence for great powers—under unipolarity, the great power—has risen.

Unipolarity implies that minor powers will not be constrained by their more powerful allies, creating possibilities for war. Disengagement by the unipole therefore induces conflicts between other states. If the unipole remains engaged, the chief problem is “recalcitrant minor powers”: states without nuclear weapons whose objectives fundamentally conflict with those of the unipole or of their neighbors. (Monteiro’s inattention to domestic politics and movement regimes prevents him from being more specific about how these objectives arise.) If the unipole pursues offensive dominance, it is likely to generate resistance from these states, and since a unipole has difficulty sending credible signals (since signaling is not sufficiently costly for it), miscalculations can lead to war. The author’s examples are the 2003 U.S.-Iraqi war and the 1999 Kosovo war between NATO and Serbia; he refers in these terms to U.S. intervention in Afghanistan after 2001, although he does not analyze it in detail.

Even if the unipole follows Monteiro’s preferred strategy of defensive dominance coupled with economic accommodation, recalcitrant minor powers have strong incentives to develop nuclear weapons and the unipole has strong incentives to stop them. War can result from these conflicts of interest, especially in view of uncertainty about the minor power’s actions and the unipole’s resolve. In the contemporary world, think of North Korea and Iran.

Theory of Unipolar Politics is carefully conceptualized and closely argued. Its great strength is that Monteiro’s structural theory helps to account both for the durability of unipolarity and the high incidence of wars involving the United States during the last quarter century. But the book has three striking weaknesses. First, it dismisses without substantial evidence the argument of John Ikenberry and others that multilateral institutions have played a major role in the durability of U.S.-led unipolarity. Second, it fails seriously to address the most compelling objection to Monteiro’s preferred strategy of economic accommodation: that China will eventually use its vast economic resources to end U.S. dominance, with a high risk of war. Third, its sanguine view of long-term Chinese strategic objectives conflicts with the premises of realism, as well as experiences with rapidly growing autocracies in the past.

Liberal institutionalists are not necessarily optimists. Indeed, some of us believe in the value of institutions for Hobbesian reasons, fearing what unconstrained states will do. Conversely, Monteiro shows that realists are not necessarily pessimists: He views major powers as able to moderate their aspirations and make cogent judgments of costs and benefits. Theory of Unipolar Politics helps us understand the durability, and war proneness, of contemporary unipolarity and presents a creative, theoretically sophisticated, and tightly argued case for what one could call optimistic realism.