Why do some regional powers collectively threatened by a potential hegemon eagerly cooperate to ensure their security, while others appear reluctant to do so? I argue that robust security cooperation at the regional level is less likely when an unbalanced distribution of power exists between the prospective security partners. In such situations, regional security cooperation tends to be stunted by foot-dragging and obstructionism on the part of materially inferior states wary of facilitating the strategic expansion of neighbours with larger endowments of power resources, anticipating that much of the coalition's gains in military capabilities are likely to be achieved through an expansion of the materially superior neighbour's force levels and strategic flexibility. Evidence drawn from primary material and the latest historiography of France's postwar foreign policy towards West Germany provides considerable support for this argument. My findings offer important correctives to standard accounts of the origins of Western European security cooperation and suggest the need to rethink the difficulties the United States has encountered in promoting cooperation among local allies in key global regions.