The past 30 years have seen the international system undergo definite change in both the nature of conflict and the way in which it is responded to by international actors. Interstate wars have largely given way to civil conflicts, which have themselves arguably changed in character over the last few decades. International responses have turned towards third-party military interventions; although not a new expression of force, the rules upon which these interventions are based have changed, and so correspondingly have their form and targets. These military incursions by states, coalitions, and International Organizations (IOs) external to the conflict have taken on a new importance, an increasingly multilateral character, and a (nominally and truly) humanitarian bent. This increasing prevalence in IO-led interventions in particular has tapered off slightly since its peak in the 2000s, but the persistence of these missions around the world speaks volumes of their continued importance. Since the conditions prompting intervention have not been eradicated, and states and IOs still appear to be invested in intervening in these situations, understanding what a successful outcome looks like and what factors influence the effectiveness of these interventions is a task that holds not only theoretical but also practical value. This dissertation contributes to this strand of literature by explicitly theorizing about what effective means, introducing the explanatory variable level of formalization, and incorporating multiple IOs into a single analysis. Because the time horizons of an intervention into an ongoing conflict and a post-conflict situation are different, the measures of what makes an intervention "effective" and the factors influencing its effectiveness will also necessarily be different. Ongoing conflicts have shorter, more urgent time frames and shorter-term goals: reducing civilian casualties and achieving victory for whichever side is supported if the intervention in biased. Post-conflict situations have less urgent time frames and longer-term goals: rebuilding physical and political/social infrastructure. Fulfilling the mission's mandate applies to both ongoing and post-conflict missions. The level of formalization of an IO varies based on the unity and clarity of the mission's command and control structures, the severity of national caveats, and the number and integration of troop-contributing countries. Essentially, higher formalization shows a unity of effort that means the IO is able to effectively utilize the resources at its disposal to create a mission that is more than the sum of its parts. Greater levels of formalization increase the likelihood of the intervention achieving its goals, the ability to quickly and decisively determine where resources are best applied and move those resources where they are needed increases the mission's ability to reduce civilian casualties, help a chosen side achieve victory, fulfill its mandates, and rebuild the host state's infrastructure. To fully develop and test this theory, this dissertation uses a mixed-methods approach, first presenting a quantitative analysis that operationalizes the key concepts and explores through simple and multiple regression the relationship between a mission's level of formalization and to what extent it achieves its goals. This time-series cross-sectional analysis shows promise, but necessitates further research. To that end, the following chapters are case studies of interventions into Bosnia-Herzegovina in the early 1990s and Kosovo starting in the late 1990s respectively. Tracing each UN, NATO, and EU mission's level of formalization across its lifespan and how that relates to the mission's outcomes is one benefit of utilizing case study research, and in this instance, shows more clearly and in greater detail how higher levels of formalization increase a mission's ability to achieve its goals, while lower levels of formalization can make a mission less effective.
