ABSTRACT
This chapter examines inter-organizational relations between the UN and Regional Organizations (ROs), from the perspective of one particular dimension to the inter-organizationalism literature, namely the interactions that occur in "nested environments." Efficiency has an intra-International Organizations (IO) dimension, positing that an inter-organizational relationship may occur in an IO's attempt to improve its input/output ratio. The UN–ROs interactions have been chosen exactly because they are the most archetypical and better researched case of nested relations in the security domain. The brief overview of the ROs' role within the UN system and the inherent clash between regionalism and universalism provides a lot of insights vis-a-vis the modus vivendi and operandi of organizations nested within broader encompassing organizations. The ROs offer financial and human resources to the UN but can also drain UN resources by developing their own intervention capacity undermining the similar UN cooperation schemes.
