Environmental issues as potential triggers of conflicts challenge the traditional definition of the concept of security focused on military threats and more generally the global system in charge of the management of international security. Indeed, born after the 2nd World War, the United Nations has been structured according to the idea of work divisions along topics; each program being responsible for one single issue. Yet, environmental security constitutes an example of a crosscutting topic that the organization intends to deal with despite its original disposition. Through data collected as a participant observer within the United Nations Environment Programme (UNEP) and Departments of Peacekeeping Operations (DPKO) and of Field Support (DFS), this paper argues that we observe a connection between the fields of international security and of environmental politics through the processes of securitization of the environment and of environmentalization of security. It shows the different modalities of these processes going beyond discourse analysis. Based on this case study, this paper also seeks to contribute to a broader reflection on the challenges of crosscutting issues in a globalized world through a redefinition of the securitization process as hybridization.