Why do some countries experience enduring, high-quality peace after ending civil war, while others experience recurring violence and make little improvement in ordinary people's lives? In this dissertation, I argue that where government implements reforms that reduce politically motivated violence against civilians, states are likely to achieve lasting, quality peace. In particular, I show that some types of security sector reforms reduce political violence against civilians, leading to both a reduction in the likelihood of conflict recurrence, and an increase in the level of post-conflict human development growth. The reduction of political vio- lence against civilians directly lowers civilian grievances against the state, and also reduces political exclusion, since opposition parties can compete and gain politi- cal power without fearing violent intimidation. The reduction of grievances limits civilians' incentives to join or support rebel movements, lowering the likelihood civil war will recur. Furthermore, when formerly excluded groups can compete for political power without fearing repression, they are more likely to actually gain power and introduce reforms that improve the well-being of socially marginalized people. I propose to test these arguments by leveraging a multi-method research design. First, I explore the causal mechanisms by conducting process tracing on the pathway case of El Salvador. After ending civil war in 1992, El Salvador implemented security sector reforms, saw an important reduction in politically motivated violence, and achieved reasonable improvement in health and educa- tion levels. The case fits into the nodes of my theoretical model, so I use process tracing to understand the causal mechanisms linking these nodes. Next, I test the theoretical model, in multiple parts, using cross-national data on a sample of civil wars ending between 1985 and 2006, finding statistical support for my theory.