Most peace operations (60%), deployed from 1978 to 2009 by international organizations (IOs), were deployed in conflicts in which other international organizations’ peace operations were active at the same time. Multiple simultaneous peace operations increased from around 10% of all peace operations in 1992 to almost 60% of all peace operations in 2007. I define two or more peace operations deployed by international organizations at the same time, in the same conflicts as multiple simultaneous peace operations (MSPOs).

This continuous increase in multiple simultaneous peace operations has not been paralleled by an increase in scholarly analysis of this phenomenon. We have little understanding of why we observe international organizations (IOs) deployed at the same time, in the same conflicts, and increasingly cooperating with each other to address these conflicts. This study asks the question: What factors determine cooperation between international organizations in peace operations?

I test several rational and social constructivist explanations introducing an innovative collective principals-multiple agents framework. There are three rational-interest explanations for inter-organizational cooperation tested: resources, complementarity, and conflict complexity. I also tested three social-constructivist arguments for inter-organizational cooperation provided by the scholarly literature: social learning, security cultures similarity, and personnel nationality similarity. This study provides the first systematic analysis of EU’s peace operations and cooperation between the EU and other international organizations. The research design is a multi-method approach, using statistical analysis from my new dataset on multiple simultaneous peace operations, data from interviews with EU officials, and case studies. There are two stages for the initiation of inter-organizational cooperation: the member-states’ acquiescence and the international organizations’ cooperation. At the first stage of cooperation, the member-states’ acquiescence is required, and the evidence provided by this study shows that member-states are acquiescent to inter-organizational cooperation. At the second stage, the findings show that international organizations cooperate with each other, when deploying peace operations, because they want to share the financial and human resources costs, and second, because they want to complement each other’s work.