Terminating Alliances: Why Do States Abrogate Agreements?

Are binding international agreements only valuable as matters stand (rebus sic stantibus), or are pacts respected in good faith regardless of changing circumstances (pacta sunt servanda)? In this article, we examine this question with respect to military alliance agreements, and we find that alliances are more likely to be abrogated opportunistically when one or more members experience changes that affect the value of the alliance, for instance a change in international power, a change in domestic political institutions, or the formation of a new outside alliance. We also find, however, that controlling for such changes, factors that affect the costs of breaking commitments (for example, democracy and issue linkage) reduce the probability that leaders will abandon their alliances in violation of their terms. We evaluate our argument empirically on a sample of bilateral alliances formed between 1816 and 1989 using competing risks duration analysis and find support for our hypotheses.