International reputation and alliance portfolios: How unreliability affects the structure and composition of alliance treaties

Why do states ever form military alliances with unreliable partners? States sign offensive and defensive military alliances to increase their fighting capabilities in the event of war and as a signal to deter potential aggressors from initiating a crisis. Yet, signing an alliance with an unreliable partner is at odds with both of these rationales. This should be particularly concerning for peace scholars and policymakers, since the uncertainty generated by unreliable partners may increase system-wide conflict. This article provides an answer to this puzzle by arguing that states continue to form alliances with unreliable partners because they can adopt rational portfolio-diversification strategies. Drawing on well-developed models from portfolio theory, we present evidence that states design their overall alliance portfolios to minimize the risks posed by allies with a reputation for being unreliable. Specifically, we show that unreliable allies are more likely to be pooled into multilateral alliances that dilute risk rather than bilateral alliances, and that states allied with unreliable partners form a greater number of alliances to hedge against the added risk of default. Together, our results demonstrate why unreliable partners may not lead to increased conflict initiation, while also providing a novel explanation for previously unexplained variation in the structure of alliance portfolios. The article contributes to the literatures on international reputation and the rational design of international institutions by demonstrating how international reputation matters in subtle and often overlooked ways