This chapter presents a neo-institutional explanation of the dynamic behind the changes in South Korea’s United Nations Peacekeeping Operations (UNPKO) mission formation. It reviews the historical development of South Korea’s overseas peace operations. The chapter discusses why competitive isomorphism is not suitable for analyzing the organizational convergence of the UNPKO units. It explores the UNPKO mission formations began to look like each other “as they respond to similar regulatory and normative pressure, or as they copy structures adopted by successful organizations under conditions of uncertainty”. The South Korean Army’s field manual defines all overseas military missions as a peace operation and categorizes them into few groups. From a competitive isomorphism perspective, “the diversity of organizational forms is isomorphic to the diversity of environments”. Organizations gain legitimacy by acting in ways that are consistent with values and norms embedded in an institutional environment.