Unanswered Threats: Political Constraints on the Balance of Power. By Randall L. Schweller. Princeton: Princeton University Press, 2006. 200p. $29.95.

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.