Alliance formation, alliance expansion, and the core

This article presents a simple cooperative game theory representation of al liance formation and expansion to counter a conventional threat along the a llies' borders. Mutual defense gains, derived from allying, arise from inte rior borders that no longer require protection. Spatial and locational attr ibutes of the allies are crucial when identifying the gains from mutual def ense and the distribution of these gains. The same number of allies can hav e vastly different cores depending on their spatial configurations. Extensi ons to the baseline case consider transaction costs, natural defenses, guer rilla warfare, and risk concerns. An application to the NATO alliance indic ates that the theory has much to say about which additional Partnership for Peace countries are likely to join NATO after the three Visegrad countries .