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States form military alliances for the security benefits they provide. Despite this common purpose of military alliances, alliances vary a great deal in their design. Notably, some states form alliances that involve extensive peacetime military coordination while others form alliances with minimal peacetime military coordination. I argue that this variation in alliance design is motivated by the bargaining power of any challengers the members face. When a state faces a relatively weak challenger it will not have an incentive to pay the greater cost associated with a high level of peacetime coordination because a low level of peacetime military coordination will deter the challenger. However, if a state faces a relatively strong challenger a low level of peacetime military coordination will fail to deter the challenger and, thus, it will be willing to pay the additional costs associated with a high level of peacetime military coordination. An empirical analysis of the design of alliances from 1816 to 2001 supports hypotheses derived from the argument. The findings highlight how the design of alliances is driven, in part, by the characteristics of nonmembers.

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.

Theories of alliance formation and war suggest that alliances influence the probability that a potential challenger will initiate a militarized interstate dispute. This is because alliances are expected to influence their members' likelihood of intervening in a potential war. More specifically, defense pacts are expected to increase the likelihood of members joining the target of a war and offense pacts are expected to increase the likelihood of members joining the initiator of a war. However, there is no empirical evidence that demonstrates that these different alliance treaty obligations have these effects on states' war intervention decisions. Therefore, in an analysis of all states' war intervention behavior in 95 wars, I provide the first estimates of these different effects of alliance agreements. After controlling for a number of measured and unmeasured factors, I find that alliances have the hypothesized effects on states' war intervention decisions. These findings provide a more comprehensive account of the effects of alliances on war intervention and offer additional support for arguments linking alliances to the onset of militarized interstate disputes.

It is well recognized that military alliances can provide their members with important security benefits. However, less attention has been paid to the policy concessions states must grant others to enter into military alliances. To study this aspect of alliances, I develop a three-actor bargaining model of alliance formation that endogenizes both external threat and policy concessions. Specifically, a target state bargains with a potential defender over the concessions it must make to ally and then responds to a potential challenger. The model suggests that what is important for policy concessions in alliances is not just the power of the threatened state but its power relative to its challenger and how an alliance will change the distribution of power. I test implications of this model using data on promised policy concessions formalized in alliance treaties and find strong support for the hypotheses. More specifically, I find that states are willing to make more concessions in exchange for an alliance when they are unlikely to defeat their challengers alone and when their allies have a large effect on their probability of winning in war. These findings refine existing theories of alliances and offer the first large-N analysis of policy concessions in alliances.

States form defensive alliances hoping to deter adversaries and avoid war. However, scholars and policy analysts often worry that if an alliance fails to deter the promise of military support will encourage escalation, pushing disputants closer to war. We show that in many cases this concern is unwarranted. We use a game-theoretic model of alliance reliability and crisis bargaining to show that the same factors that indicate unreliability and provoke disputes also encourage alliance members to make concessions rather than risk war. We test this hypothesis using a sample of militarized disputes initiated against members of defensive alliances, where recent shifts in military capabilities represent changes in challengers' estimates of alliance reliability. Less-reliable alliances are less likely than reliable allies to deter disputes, but they also decrease the probability of escalation relative to reliable alliances. Unreliable alliances need not encourage war; rather, they can discourage it.

States form defensive alliances hoping to deter adversaries and avoid war. However, scholars and policy analysts often worry that if an alliance fails to deter the promise of military support will encourage escalation, pushing disputants closer to war. We show that in many cases this concern is unwarranted. We use a game-theoretic model of alliance reliability and crisis bargaining to show that the same factors that indicate unreliability and provoke disputes also encourage alliance members to make concessions rather than risk war. We test this hypothesis using a sample of militarized disputes initiated against members of defensive alliances, where recent shifts in military capabilities represent changes in challengers' estimates of alliance reliability. Less-reliable alliances are less likely than reliable allies to deter disputes, but they also decrease the probability of escalation relative to reliable alliances. Unreliable alliances need not encourage war; rather, they can discourage it.

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.

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.