The article represents an attempt to integrate the analysis of threats to national security of Slovenia with its activities to join NATO. Security threats are the key catalyst of reforms in NATO and Slovenian national defencesystem. In this respect, NATO has adapted to the new security environment after the end of cold war with its concept of double enlargement and Slovenia faced the fact of being unable to assure its national security completely alone in the light of increasing transnationalization of security threats. The authorćs main idea is that exactly the wish to join NATO has brought to intensive reforms within the national defence and national securitysystem. It represented a catalyst for improving both systems and, therefore, contributing to Slovenian national security.