Outsourcing Security: Alliance Portfolio Size, Capability, and Reliability

The overexpansion of alliance portfolios can diminish the overall security of states. Due to the fear of abandonment, states have an interest in expanding the size and capabilities of their alliance portfolio to ensure the receipt of adequate assistance in the event of a conflict. However, each ally's incentive to intervene—their expected reward—decreases as alliance portfolios become larger and more powerful. In such situations, states’ efforts to address the alliance abandonment problem may serve to exacerbate it. Hypotheses regarding the influence of alliance portfolio size and capabilities on conflict intervention are tested. Analysis suggests that states must possess some minimal threshold of military capability before an expanded alliance portfolio increases the likelihood of intervention in conflict. Furthermore, states must be mindful of their individual capabilities relative to those of their collective alliance portfolio. States appear capable of adding roughly 1.5 times their own capability through alliances before additional expansion actually decreases the probability of intervention.