Since 1945, violent conflict has occurred primarily within sovereign states rather than among them. These internal conflicts have far surpassed international conflicts in lethality, economic destruction, and social upheaval. This phenomenon is diverse: no region has avoided civil wars, while the stated aims of rebel groups have ranged widely. Prominent examples include anti-colonial nationalists in Algeria, Mozambique, and Kenya; ethnic separatists in Eritrea and Bosnia; leftists in Latin America and Southeastern Asia; Islamic fundamentalists in Afghanistan, Iraq, and Syria; and income seeking warlords in Liberia and Sierra Leone. Internal conflicts have emerged in rich European countries such as the United Kingdom and Spain, and in the context of state collapse and extreme poverty in the Democratic Republic of Congo and Somalia. Some civil wars have lasted only weeks, while the longest-- in Sudan-- lasted over 40 years.Intense violent conflicts often leave core state institutions debilitated, fragmented, or, in some cases, totally destroyed. For these societies, the central tasks for ending conflict and beginning post-war recovery involve reinvigorating or reestablishing legitimate state authority. These post-war states must both win the acquiescence of the governed and develop the infrastructural power to implement state policy. The risks of conflict relapse are significant: since 1970, 44 of 111 post-war cases (40 percent) relapse into a full-fledged civil war, while 68 of 111 (61 percent) experience at least a low-level conflict. The time for policymakers to mitigate this risk is short: of post-war countries that fall back into civil war, the median time to relapse is just 35.5 months. The immediate post-war environment is therefore particularly critical for determining the political, economic, and social trajectories of conflict-affected countries. The right combination of policies can help determine whether a country recovers quickly and secures any available peace dividend, or whether it relapses and slides into a conflict trap. This dissertation explains how societies that have managed to end their civil wars are able or unable to rebuild political order in the their post-war period.This dissertation focuses on one key policy arena-- perhaps the most critical policy arena-- for post-war societies to address: the security sector. It may sound simplistic or even tautological to claim that the organization, disposition, control, and reform of armed groups are the most important task for a post-war society to undertake. It may seem obvious to stress the importance of the size, competencies, oversight, social embeddedness, and other qualities of the military, the police, the intelligence services, and any remaining armed non-state actors. But such qualities resist easy quantification, and most scholars and practitioners over the past decade have focused on economic performance, political democratization, communal reconciliation, post-conflict justice, and other “soft-power” variables to explain patterns of post-war successes and failures. The following chapters attempt to shift the conversation back to the formation and reformation of security sector actors in war-affected countries.