Armed non-state groups frequently cooperate with other groups, even to the point of allyingwith them. This dissertation examines the role of group leaders in understanding when and to what extent do armed groups cooperate, and the impact that cooperation has on the severity of terrorist violence. When operating in complex environments such as civil wars, armed groups face information barriers in credibly signaling to other groups about their willingness to engage in cooperation. I argue that group leader’s length of time in power and their prewar experiences help resolve the information uncertainty and promote cooperation between armed groups. Prior to assuming power, some leaders participate in a variety of military and political activities. These prewar leader experiences, I argue, condition the type of armed group cooperation that occurs during civil wars. Furthermore, I also explore if and how cooperation influences the violent behavior of armed groups. Specifically, I argue that in addition to the number of cooperative ties, the diversity and directionality of support within those ties also matter equally, if not more, in affecting the severity of armed group violence. Using a large-N design, I conduct a variety of statistical models to test my hypotheses related to the incidence, type, and effects of cooperation among armed groups. My findings show that leader tenure is positively and statistically significantly associated with the occurrence of armed group cooperation. I also find that leaders with prior military experience are more likely to engage in informal ties while prior political experience increases the odds of formal cooperation in a statistically significant manner. Lastly, the proportion of cooperative ties in which an armed group is the major provider of support is shown to have a negative and statistically significant impact on violence lethality. Overall, this study demonstrates the value of leaders in understanding the nature of armed group interactions. Cooperative ties involving armed groups are not necessarily uniform and it is recommended that both scholars and policy makers should consider the heterogeneity in ties when evaluating the risks posed by armed groups.