Defense Cooperation Agreements and the Emergence of a Global Security Network

Bilateral defense cooperation agreements, or DCAs, are now the most common form of institutionalized defense cooperation. These formal agreements establish broad defense-oriented legal frameworks between signatories, facilitating cooperation in such fundamental areas as defense policy coordination, research and development, joint military exercises, education and training, arms procurement, and exchange of classified information. Although nearly a thousand DCAs are currently in force, with potentially wide-ranging impacts on national and international security outcomes, DCAs have been largely ignored by scholars. Why have DCAs proliferated? I develop a theory that integrates cooperation theory with insights from social network analysis. Shifts in the global security environment since the 1980s have fueled demand for DCAs. States use DCAs to modernize their militaries, respond to shared security threats, and establish security umbrellas with like-minded states. Yet, demand alone cannot explain DCA proliferation; to cooperate, governments must also overcome dilemmas of mistrust and distributional conflicts. I show that network influences increase the supply of DCAs by providing governments with information about the trustworthiness of partners and the risk of asymmetric distributions of gains. DCAs become easier to sign as more states sign them. I identify two specific network influences—preferential attachment and triadic closure—and show that these influences are largely responsible for the post-Cold War diffusion of DCAs. Novel empirical strategies further indicate that these influences derive from the proposed informational mechanism. States use the DCA ties of others to glean information about prospective defense partners, thus endogenously fueling further growth of the global DCA network.