冷战结束以来，美国盟友与第三方国家爆发过多次国际危机，但美国在历次危机中的行为选择却存在明显差异。例如，同样是领土和主权争端，美国在2002年摩洛哥与西班牙爆发的佩雷希尔岛危机中选择进行政治调解，但在2008年柬埔寨和泰国爆发的柏威夏寺争端中却没有采取任何行动；同样是土耳其和叙利亚之间爆发的危机，美国在1998年的危机中仅呼吁双方保持克制，但在2012年的危机中却对土耳其提供了军事支持。那么，美国应对盟友危机的潜在方式有哪些？什么因素会影响美国对于危机应对方式的选择？美国在选择危机应对方式时又会遵循什么逻辑？本文旨在就这些问题展开研究。既有研究从体系结构、联盟需求、联盟及其成员国属性三个角度直接或间接分析了美国在盟友卷入国际危机时的行为选择及其背后逻辑，但并未提出系统的解释框架。在吸收借鉴既有研究的基础之上，本文将美国应对盟友危机的方式区分为了支持盟友、反对盟友、调解、不介入四种类型，并以美国进行霸权护持的战略需求作为理论起点，提出危机是否以及如何挑战了地区安全秩序，还有当事盟友的议价能力，共同决定了美国对于危机应对方式的选择。具体而言，危机是否挑战了地区安全秩序会决定美国介入危机的必要性；如果不构成实质挑战，美国更可能选择不介入。而在危机对地区安全秩序构成实质挑战的情况下，盟友的行为是否符合美国的秩序偏好反映了危机挑战秩序的过程，会影响美国对具体介入方式的选择；如果符合，美国更可能选择支持盟友，反之则更倾向于反对盟友。当事盟友的议价能力同样会影响美国对具体介入方式的选择。如果议价能力强，美国更可能支持盟友；如果弱，美国支持盟友的意愿会有所下降。通过对美国在印度—巴基斯坦之间的7场危机、叙利亚—土耳其之间的3场危机、中国—菲律宾之间的2场危机这三组案例中的行为选择进行比较分析，本文整体上验证了理论机制的有效性，而对案例集中其余案例的补充分析以及与竞争性解释的比较进一步证明了理论机制的解释力。在此基础上，研究发现，除了盟友发展或使用核武器的情况外，美国几乎不会在危机中采取反对盟友的行为，并且美国对盟友危机进行调解的可能性越来越低，这也为中国应对潜在的国际危机以及美国的介入提供了政策参考。

Since the end of the Cold War, numerous international crises involving US allies and other countries have occurred, each met with distinctly varied responses from the US. For instance, in the 2002 sovereignty dispute between Morocco and Spain, the US pursued political mediation, whereas it remained uninvolved in the 2008 sovereignty dispute between Cambodia and Thailand. Similarly, during the 1998 crisis between Turkey and Syria, the US merely urged restraint from both parties, yet it provided military support to Turkey during a similar crisis in 2012. Thus, what are the potential courses of action available to the US in responding to an international crisis involving its ally? What factors influence the US’ decision-making in such crisis? What logic does the US follow when choosing its response to a crisis? This paper seeks to address these questions. Existing studies have examined the US’ behavioral choices in ally-involved international crises through three primary lenses: system structure, the needs for alliance, and the characteristics of the alliances and their member states. While these perspectives offer valuable insights, they fail to comprehensively address the research questions posed in this paper.Building upon previous research, this paper identifies four main behavioral options for the US in responding to crises involving allies: support for ally, opposition to ally, mediation, and non-intervention. Grounded in the strategic need of preserving US hegemony, this paper argues that whether and how the crisis challenges the regional security order, coupled with the bargaining power of the ally embroiled in the crisis, jointly determine the US’ response. Specifically, whether the crisis challenges the regional security order will determine the necessity of US intervention in the crisis. If not, the US is more inclined towards non-intervention. Conversely, if the crisis challenges the regional security order, whether the ally’s behavior conforms to the order preference of the US reflects the process of the crisis challenging the order, and will affect the choice of the US. If so, the US is more likely to choose support for ally; if not, it is more inclined to choose opposition to ally. Furthermore, the bargaining power of the ally also shapes the US’ decision-making. When an ally wields significant leverage against the US, the likelihood of US support increases; conversely, weaker bargaining power diminishes US willingness to intervene.Through a comparative analysis of US responses in three sets of crises—seven cases between India and Pakistan, three between Syria and Turkey, and two between China and the Philippines—this paper validates the proposed theoretical framework. Additional examination of other cases and comparison with alternative explanations further reinforce the explanatory power of this framework. Notably, except in scenarios involving the development or deployment of nuclear weapons by allies, the US seldom opposes its allies in crises. Moreover, US mediation in ally-involved crises appears increasingly improbable. These findings offer valuable policy insights for China in managing potential international crises and navigating US intervention.