This paper analyses the effects of an on-going decline of American hegemony on patterns of institutionalized cooperation between the U.S. and its main allies. I argue that a relative decline in a hegemon’s power may cause it to shift its preference away from formal multilateral cooperation towards ad hoc, informal cooperation in order to enhance flexibility and reduce the costs of leadership, thereby increasing ‘returns to power’. Based on an analysis of the changing design and internal workings of major multilateral security institutions from the mid-1980s to the present, the paper observes that as emerging powers have become more assertive participants in the global institutional order, the U.S. has increasingly favored cooperation through ad hoc 'coalitions of the willing' that are narrower in scope and membership, lighter on formal decision-making apparatuses, and richer on flexibility than traditional multilateral security organizations. As the U.S. finds it is often unable to 'lock-in' its preferred policy outcomes within inclusive multilateral security institutions, it has embraced a new form of ad hoc and discriminatory multilateralism, which uses the power of 'privileged clubs' to promote American policies. This has had visible implications for the way the U.S. coordinates security policies with its main allies in Europe and East Asia, especially in the areas of non-proliferation policy and out-of-area interventions.
