This dissertation develops a novel theory of tribal alignments in civil wars, focusing on both their initial formation and subsequent realignment. Tribes are essential stakeholders in many societies that experience civil wars, including Iraq, Afghanistan, Syria, Libya, and Yemen. Despite their prominent role, these actors are overlooked in the scholarly literature on civil wars. Drawing on their distinct internal hierarchies, cultural norms, and clientelist relationships, tribes respond to civil war dynamics in ways that diverge from other non-state actors. Why do some tribes align with the state while others support rebel forces? Why do some shift their allegiances during conflict while others remain consistent? This dissertation argues that tribal alignments are driven by a logic of threat perception: tribes strategically align with the actor they perceive as the lesser evil in terms of threatening their social structure and clientelist relations. Specifically, two mechanisms shape their decisions. First, tribes assess threats to their internal order, particularly challenges to the authority of sheikhs and the disruption of traditional norms. Second, they evaluate the security of their external clientelist networks, which are vital for material resources and political relevance. When either the state or rebel forces jeopardize these foundations, tribes align to preserve cohesion and leadership. As threat perceptions evolve, tribal alignments remain fluid, shifting when former adversaries become the safer option. The argument is substantiated by original fieldwork, including 40 in-depth interviews conducted in Iraq and Jordan. An in-depth with-in case study and a comparative case study of three tribes demonstrates patterns and variations in tribal alignment strategies. This research contributes to civil war studies by theorizing tribes as autonomous political actors with their own strategic logic, distinct from both insurgents and civilians. It offers a new framework for understanding alignment behavior in civil wars, and underscores the role of non-material factors—such as honor, hierarchy, and tradition—in shaping wartime decisions. The findings carry practical implications for conflict mediation and post-conflict reconstruction, suggesting that effective peacebuilding must engage with, rather than bypass, tribal authority structures.