Alliance Formation, Alliance Expansion, and the Core

This article presents a simple cooperative game theory representation of alliance formation and expansion to counter a conventional threat along the allies’ borders. Mutual defense gains, derived from allying, arise from interior borders that no longer require protection. Spatial and locational attributes of the allies are crucial when identifying the gains from mutual defense and the distribution of these gains. The same number of allies can have vastly different cores depending on their spatial configurations. Extensions to the baseline case consider transaction costs, natural defenses, guerrilla warfare, and risk concerns. An application to the NATO alliance indicates that the theory has much to say about which additional Partnership for Peace countries are likely to join NATO after the three Visegrad countries.