Power changes, alliance credibility, and extended deterrence

A primary motivation for forming military alliances is to deter adversaries. However, some alliances are more effective at deterrence than others. Deterrence theory suggests that an alliance may fail to deter if the commitment is not considered credible by adversaries. Building on alliance reliability research, we contend that alliances whose members have experienced significant changes in power since formation are vulnerable to violation. Moreover, we argue that these changes are observable to adversaries, making them more likely to target the alliance. We test this argument using a sample of countries with membership in defensive alliances between 1816 and 2000. Our results indicate that states whose allies have experienced significant changes in power are more likely to be the target of militarized disputes than states whose allies have not experienced significant changes in power. These findings connect alliance reliability research to deterrence theory, illustrating the need to take into account changes over time in understanding the deterrence potential of military alliances. Our findings also suggest that future research should take these processes into account in studying dispute escalation and intra-alliance relations.