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

In this insightful and elegantly written book, Randall Schwellerexamines the phenomenon of underbalancing, which he defines assituations in which “threatened countries have failed to recognize aclear and present danger, or, more typically, have simply notreacted to it or, more typically still, have responded in paltry andimprudent ways” (p. 1). The study is motivated by the failure ofmany states throughout history to act in accordance with thecardinal prediction of structural realist theory that states willtend to balance against rising powers that threaten their survival,through the acquisition of arms and/or allies. Schweller argues thatdomestic political factors account for this discrepancy, enhancingor diminishing the ability and/or willingness of states to mobilizetheir national resources in response to systemic dangers.