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.

各国结成防御联盟，希望威慑对手并避免战争。然而，学者和政策分析人士经常担心，如果联盟未能阻止提供军事支持的承诺，将助长事态升级，将争端推向战争。我们表明，在许多情况下，这种担忧是没有根据的。我们使用联盟可靠性和危机讨价还价的博弈论模型来表明，表明不可靠和引发争端的相同因素也会鼓励联盟成员做出让步，而不是冒险开战。我们使用针对防御联盟成员发起的军事争端样本来检验这一假设，其中最近军事能力的变化代表了挑战者对联盟可靠性估计的变化。不太可靠的联盟比可靠的盟友更不可能阻止争端，但相对于可靠的联盟，它们也降低了升级的可能性。不可靠的联盟不必鼓励战争；相反，他们可以阻止它。