To what extent can international peacekeeping promote micro-foundations for positive peace after violence? Drawing on macro-level peacekeeping theory, our approach uses novel experimental methods to illustrate how monitoring and enforcement by a neutral third party could conceivably enhance prosocial behavior between rival groups in a tense, postconflict peacekeeping environment. Using a laboratory experiment in postwar Kosovo, we find that third-party enforcement is more effective at promoting norms of trust between ethnic Serbs and Albanians than monitoring alone or no intervention at all. We then consider real-world extensions for building positive peace across different intervention environments. Using a dictator experiment that exploits heterogeneity in NATO peacekeeping in different regions of Kosovo, our inferences about monitoring and enforcement appear robust to ecological conditions in the field.