Peacekeepers are mostly deployed in conflict or post-conflict environments where violence is either ongoing or lingering. Thus, violence remains a priority for peace missions. Consequently, peacekeeping is deemed successful or effective according to whether it curbs conflict in several dimensions. Effective missions are those responsible for decreasing the intensity of battle violence, protecting civilians, and containing conflict diffusion and recurrence in the postwar phase. Each mission, however, is deployed in different contexts and operates under variable conditions that affect the operation’s capacity to influence conflict. Concerning mission features, peacekeeping success is more likely when large contingents are deployed under robust mandates. Mission type, size, and composition signal credible commitment from the international community and empower peacekeepers to halt violence while guaranteeing the implementation of peace agreements. These nuanced understandings of peacekeeping stem from the availability of new data on both conflict and peace operations at the national and subnational levels of analysis. Moreover, the empirical study of the effectiveness of peace operations has recently been flanked by simulation-based forecasting, field experiments, and surveys investigating local-level outcomes of peace missions.

Unsurprisingly, the focus on violence and conflict outcomes as indicators of success is debatable. First, in dealing with violence, peacekeeping operations produce spillover effects that are largely neglected, such as refugee flows and terrorist violence. Second, given the wide range of functions performed by UN peacekeepers, including electoral assistance, economic reconstruction, and state building, it is reasonable to include these aspects when defining effectiveness. Third, and relatedly, no assessment of short- versus long-term implications of peacekeeping for political, social, and economic development in the host country has been forthcoming. While reducing infant mortality, inequality, and crime are not necessarily tasks for peacekeepers, it is vital to study whether and how UN missions may have shaped the quality of peace in host countries.