Across four self-contained essays, this dissertation seeks to identify which features make international organizations (IOs) effective peacemakers in modern civil wars. The first essay introduces an original dataset on the institutional design of 21 peace-brokering IOs between 1945 and 2010. The second essay contains a statistical study of 122 IO civil war mediation episodes, examining how variation in institutional design affects outcomes. The third essay presents an in-depth case study, comparing interventions by the Arab League and the United Nations in Syria in 2011 and 2012. The fourth essay is a statistical examination of how IO member state biases influence mediation effectiveness. Overall, this dissertation demonstrates that the performance of peace-brokering IOs cannot be accurately evaluated without taking institutional variation into account. IOs display considerable heterogeneity in de­sign and capabili­ties and this variation has implications for the nature and effectiveness of IO interventions. Quantitative evidence reveals that IOs with strongly centralized instruments for supporting mediation and, in particular, peacekeeping operations are more likely to end civil wars. Qualitative evidence shows that IOs with such capabilities can engage in interventions of greater scope and credibility, enhancing their ability to shape the calculations of civil war disputants. Combined, the studies suggest that although institutional capabilities are necessary for sustained intervention effectiveness, they are conditioned on other organizational attributes. IOs with high preference homogeneity can signal intervention durability, giving them an edge over IOs with divided memberships. IOs that contain member states that have pro­vided direct support to civil war disputants outperform IOs that lack such member states. / <p>This dissertation consists of four self-contained essays dealing with different aspects of conflict management by international organizations.</p><p>Essay 4 previously appeared in 2014 as “Leanings and dealings: Exploring bias and trade leverage in civil war mediation by international organizations” (<em>International Negotiation, 19</em>(2), 315–342).</p>