What accounts for overarching trajectories of civil wars? This article develops an account of civil war as a social process that connects dynamics of conflict from pre- to post-war periods through evolving interactions between nonstate, state, civilian, and external actors involved. It traces these dynamics to the mobilization and organization of nascent nonstate armed groups before the war, which can induce state repression and in some settings escalation of tensions through radicalization of actors, militarization of tactics, and polarization of societies, propelled by internal divisions and external support. Whether armed groups form from a small, clandestine core of dedicated recruits, broader networks, social movements, and/or fragmentation within the regime has consequences for their internal and external relations during the war. However, not only path-dependent but also endogenous dynamics shape overarching trajectories of civil wars. During the war, armed groups develop cohesion and fragment in the context of evolving internal politics, including socialization of fighters, institution-building in the areas that they control, which civilians can collectively resist, competition and cooperation with other nonstate and state forces, and external influence. After the war, armed groups transform to participate in continuing conflict and violence in different ways in interaction with multiple actors. This analysis highlights the contingency of civil wars and suggests that future research should focus on how relevant actors form and transform as they relate to one another to understand linkages between conflict dynamics over time and on continuities and discontinuities in these dynamics to grasp overarching trajectories of civil wars.