This chapter empirically investigates the link between reputation and cooperation among nations, especially in cases of security alliance formation, which are especially fraught and high-stakes processes for nations. Specifically, the focus here is on testing the argument that when states develop reputations for competence in cooperative situations, they are more likely to experience cooperation in other future interstate affairs. These findings provide support for the conclusion that, when nations seek alliance partners, they pay close attention to the past alliance-related behavior of their potential partners with other states. Specific, historical instances of Anglo-German and Anglo-Japanese alliance formation clarify the arguments of this chapter.