In this dissertation, I examine how international law on the use of force influences the behavior of leaders in international crises. I argue that leaders are less likely to escalate militarily in international crises when the Charter of the United Nations and related legal principles prohibit the use of force compared to when international law allows for the right of self-defense. I argue that international law can constrain crisis actors from employing the large-scale use of force by facilitating the dynamics of reciprocity in crisis-bargaining. Crisis actors who act in accordance with international law can expect to receive greater international support, while actors that violate the law can expect to obtain less support. International law therefore promotes the peaceful resolution of international crises because actors with the support of third parties can credibly signal their intent to employ the use of force in self-defense and deter their adversaries from engaging in the aggressive and illegal use of force in the first place. I find strong support for my theoretical argument using both quantitative and qualitative methods. Using an original dataset on international law on the use of force in international crises from 1946-2005, I find that leaders are less likely to escalate militarily when international law prohibits the use of force than when they have a right to use force. I also find that intergovernmental organizations are more likely to support leaders who have the right to use force, providing support for the underlying causal mechanism in my argument. Finally, I present a case study of the Cuban Missile Crisis and find that international law contributed to President Kennedy's decision to implement the blockade, instead of employing air strikes against Cuba.