External threat and alliance formation

Scholars generally believe that external threats drive military alliances. However, existing statistical studies of alliance formation fail to find a consistent relationship between the two. In this research note, I argue that this is because they do not correctly proxy for the existence of an external threat. Previous studies employ measures based on past militarized disputes, but a valid measure must capture expectations of future militarized disputes. To identify a better indicator of external threat, I situate alliance formation in crisis bargaining theory. The framework suggests that a target will be more likely to seek an alliance as its challenger's probability of winning in war increases. I test this hypothesis and find a positive relationship between external threat and alliance formation. My analysis provides support for a central pillar of alliance theorizing. Additionally, it suggests that any pacifying effects of alliances may be difficult to uncover, as alliances form when the probability of conflict is already high.