What happens to alliances when their precipitating threats disappear? Understood in realist terms, alliances should not outlive the threats they were created to address. As coalitions of states aggregating their capabilities to cope with common enemies, alliances should have no purpose beyond deterrence or defense, and no resources beyond the power and purpose of their members. 1 When threats disappear, allies lose their reason for cooperating, and the coalition will break apart. Consistent with the theoretical underpinnings of realist theory, early in the post-Cold War period many scholars predicted NATO's demise. 2 Yet a decade after the disappearance of the Soviet threat, NATO still exists. To explain the persistence of NATO, we must rst accept that alliances are not always merely aggregations of national power and purpose: they can be security institutions as well. As institutions, alliances themselves make a difference in the capacity of states to coordinate their policies and mount credible deterrence or defense. 3 Institutions persist because they are costly to create and less costly to maintain, so they may remain useful despite changed circumstances. 4 However, this institutionalist argument is incomplete. The concept of ''sunk costs'' alone does not tell us when institutions will persist-clearly, many institutions (such as the Warsaw Pact) do not.In this article I develop an explanation for institutional adaptation to explain variation in institutional persistence. An institution will not persist if it no longer serves the interests of its members, and so alliances predicated only on threats are unlikely to survive when the threats disappear. But unlike mere alignments of states, security I thank Robert Art, Bat Batjargal, Jeffrey Checkel, Richard Falkenrath, Peter Gourevitch, Robert Keohane, David Lake, Lisa Martin, and two anonymou s reviewers for their comment s on earlier versions of this article. I am especially indebted to Keohane for our work together on security institutions over several years, without which I could not have developed the ideas and research that led to this article, and to Jim Goldgeier for generously sharing his knowledge and contacts on NATO.

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The role of Western governments in the disintegration of the Soviet Union was complex. The two most important factors that undermined the Soviet state were the deepening economic chaos under Mikhail Gorbachev and the rapid growth of internal political dissent. Western policies tended to magnify both of these factors. This is not to say, however, that Gorbachev's original decision to embark on an economic reform program was simply the result of pressure created by Western defense spending and military deployments. The Soviet economy was plagued by severe weaknesses, of which the misallocation of resources and excessive military expenditures were only a small part. Gorbachev's initial economic reforms were spurred by his awareness of the country's general economic problems. After the first round of reforms failed, he sensed that arms control and reductions in military spending would be helpful for the next stage. Even so, the belated cuts he made in military spending (beginning in 1990) were of relatively little consequence. The West's refusal to pour money into the Soviet system without evidence of structural reform in the last years of the Soviet regime, and Western pressure on Gorbachev not to crack down on political dissent and separatism, did hasten the Soviet collapse. These policies denied the Soviet system resources that might have prolonged its survival, and they helped to deter Gorbachev from using decisive force against elements that were splitting the Soviet Union apart.