|Abstract:||This dissertation explores the impact of legitimacy concerns on the strategic decision making of separatist rebel groups. Since the end of the Cold War, the number of new states in the international system has increased consistently. Many of these have risen from the ashes of states that have collapsed under the weight of their own domestic political and economic institutions as well as widespread popular support for separation, as happened in the former Soviet Union and Yugoslavia. Others feature populations and area too small to be deemed consequential in the international system, and have been peacefully granted independence through international agreements, as happened in places like Micronesia. A few, however, earned statehood on the back of blood and hard-fought self-determination, at times aided by important members of the international system. These conflicts are among the few examples of separatist conflicts that met with success in establishing a new state. Despite the fact that separatist conflicts make up roughly a third of all ongoing armed conflicts, only four – Namibia, Eritrea, East Timor, and South Sudan – have resulted in statehood in the post-Cold War era. This statistic reflects the difficulty in establishing new states in an international system that prioritizes the norm of territorial integrity over that of the right to self-determination. In this dissertation, I argue that the difficulty of establishing new states is not lost in separatist actors, and that they conduct their struggles in ways strategically designed to endear themselves to important gatekeepers of statehood – namely, their domestic population and important members of the international community. They do this by trying to manufacture legitimacy – that is, their right and ability to govern a people and rule over a piece of territory. I argue that, owing to concerns of legitimacy, we can observe systematic differences in the way separatist actors conduct warfare compared to other rebel groups, and these differences usually manifest in ways that avoid legitimacy costs and procure legitimacy gains. Indeed, based on a quantitative analysis of separatist and non-separatist conflicts in the post-Cold War era, it appears that separatist rebel groups engage in lower levels of violence and employ fewer economic forms of civilian targeting, such as civilian killing, use of child soldiers, sexual violence, kidnapping, theft, and seizure of humanitarian aid. I also find that separatists are more likely to engage in practices that are intended to demonstrate the efficacy of their political institutions, such as extortion – or, in other words, taxation without legality. It also appears, however, that legitimacy concerns are only one piece of a complicated puzzle. In a micro-level analysis making use of novel month-level data, I find that developments within conflicts, such as significant battlefield losses, can cause groups to re-think their strategies, and may lead them to target more civilians in order to take advantage of short-term benefits when negative strategic shocks leave their potential for long-term success in doubt. Taken together, these findings contribute novel insights to the scholarly discussions surrounding civilian targeting in armed conflicts in general, separatism in particular, as well as state-building, legitimacy, and the micro-dynamics of civil war. They also provide useful insights for conflict-management practitioners seeking viable means of lessening the suffering of non-combatants caught up in the charnel house of civil wars.