In this insightful and elegantly written book, Randall Schweller examines the phenomenon of underbalancing, which he defines as situations in which “threatened countries have failed to recognize a clear and present danger, or, more typically, have simply not reacted to it or, more typically still, have responded in paltry and imprudent ways” (p. 1). The study is motivated by the failure of many states throughout history to act in accordance with the cardinal prediction of structural realist theory that states will tend to balance against rising powers that threaten their survival, through the acquisition of arms and/or allies. Schweller argues that domestic political factors account for this discrepancy, enhancing or diminishing the ability and/or willingness of states to mobilize their national resources in response to systemic dangers.

Specifically, Schweller identifies four variables that shape the balancing behavior of a given state. These are elite consensus, elite cohesion, government/regime vulnerability, and social cohesion. The first two variables affect a state's willingness to balance, while the latter two affect its ability to mobilize the national resources necessary to balance. In short, he hypothesizes that a state will be more susceptible to underbalancing if its political elites are divided about the source and urgency of the threat, its political leadership is fragmented by internal fissures, its government is politically weak and highly vulnerable to electoral or violent deposition, and its society is rent by deep social divisions. By contrast, a state that possesses a united and cohesive political elite, a politically secure regime, and an integrated society will be more likely to balance threats effectively. Schweller adds that if such favorable conditions obtain in the absence of an external threat, the result will not be defensive balancing behavior but, rather, opportunistic expansionism.

The theory is put to the test in a series of succinct and well-crafted case studies: interwar Britain and France; France from 1877 to 1913 (which is divided into two parts, 1877–98 and 1898–1913); and the War of the Triple Alliance (1864–70) launched by Paraguay against the much stronger countries of Brazil, Argentina, and Uruguay. In the cases of interwar Britain and France, and France between 1877 and 1898, domestic incoherence resulted in underbalancing against the threat posed by Germany. Conversely, in pre–World War I France, and Paraguay during the middle to late 1860s, domestic coherence resulted in optimal balancing against Germany, in the former case, and an aggressive war of expansion against the internally divided regional powers of Argentina and Brazil, in the latter case.

Schweller's theory is not only compelling but also impressive in scope. Not only does it explain “variation across space and time in state responses to threats” (p. 47), but it also explicates the conditions under which states will be most likely to engage in opportunistic aggression. Most broadly, it accounts for the relative decline in frequency of interstate aggression since the golden age of the European balance of power in the seventeenth and eighteenth centuries, owing to the gradual replacement of highly coherent absolute monarchies with more pluralistic and inclusive polities. This insight segues into a fascinating discussion of the only major exceptions to this historical trend, the fascist powers of post–World War I Germany, Italy, and Japan. According to Schweller, these regimes were only able to achieve a level of internal coherence comparable to the great powers of early modern Europe by infusing their realpolitik with a virulent racist ideology.

Inevitably, like all other ambitious and important works, Unanswered Threats raises some critical questions and comments. First, Schweller's theory of underbalancing is cumbersome, comprising four independent variables, which are then incorporated into five causal schemes, some of which proceed to incorporate additional intervening variables drawn from the fields of sociology and motivational psychology. This raises the question of whether the author has sacrificed too much theoretical parsimony in the pursuit of excess explanatory leverage.

Second, Schweller does not provide clear coding rules for his theory's independent variables, which makes them difficult to operationalize in new cases. For example, with regard to the elite consensus/disagreement variable, how would one code the U.S. political elite on policy toward the USSR during the Cold War? For much of this period, a broad political consensus existed in favor of pursuing a grand strategy of containment toward the Soviet Union. However, bitter disagreements repeatedly emerged both within and between the two major political parties on the precise tactics that should be used in the pursuit of that strategy.

Third, the sole empirical evidence Schweller delivers of prudent and effective balancing is French policy vis-à-vis Germany in the years immediately preceding World War I. This case is somewhat troublesome, though, since France's military doctrine at the time was offensive and provocative in orientation, not defensive, and therefore likely contributed to the outbreak of war.

Finally, there appears to be a contradiction between Schweller's theory and some of his evidence. On the one hand, it posits that the more internally coherent the state, the more likely that state will be to respond effectively to threats and opportunities presented by the international system. On the other hand, however, the author introduces as empirical evidence the cases of Paraguay during the War of the Triple Alliance, and, in the book's concluding chapter, the twentieth-century fascist states. All of these cases consist of highly coherent autocratic states attempting to buck the international system by engaging in reckless overexpansionism. This evidence suggests that rather than being most apt to engage in prudent and self-preserving balancing behavior, the most coherent states have been inclined to pursue the opposite course of imprudent and self-destructive aggression. Conversely, those great powers that, over the long term, have been most adept at balancing power and least susceptible to overexpansion are the considerably less-coherent—but geopolitically blessed—liberal democracies of Britain (excepting the middle to late 1930s) and the United States.

In sum, Unanswered Threats represents a significant contribution to the burgeoning theoretical paradigm of neoclassical realism, whose works share in the belief that domestic political factors serve as critical intervening variables between the pressures and opportunities generated by the international system, and states' foreign policy responses to those pressures and opportunities. In it, Schweller identifies a critical gap in the explanatory power of structural realism, and proposes an intriguing theory incorporating various domestic political variables to fill that gap. However, his theory of underbalancing is problematic insofar as it sacrifices a considerable amount of theoretical rigor in the effort to account for variant state responses to rising threats, even as it continues to leave unanswered the pivotal question of which states will be most likely to engage in prudent and effective balancing behavior. Thus, the book is a formidable first cut on the salient topic of underbalancing, but will likely not be the final one.