ESSI

We develop a framework of supranational security cooperation in which countries dif fer in the benefits they derive from collective protection and form expectations about the participation of others. Building on the economics of alliance burden sharing and a prospective model of coalition formation, we characterize stable partial coalitions and show how changes in the external threat can alter the set of sustainable coalition structures.

Country-specific benefits depend jointly on population, economic size, and geographic exposure, allowing a common security environment to generate heteroge neous incentives to participate. We apply the framework to the European Sky Shield Initiative (ESSI). The observed ESSI membership does not satisfy the model’s stability conditions, whereas alternative configurations that include Italy, France, or both can be stable over distinct ranges of the threat parameter.