The accession of the Republic of Croatia to the Schengen Agreement and the full implementation of its acquis communautaire at its borders was a long and demanding process. A multitude of authors at national, as well as European level were involved in this process. This thesis deals with the issue of national security of the Republic of Croatia and the security of its borders after entering the Schengen area. For this purpose, the pre-entry situation at European and national level, is described. After two major crises – the migrant crisis of 2015/2016 and the crisis caused by the pandemic, the entire Schengen system remained shaken and its future was uncertain. It was after these events that the leadership was persistent in further expanding the borderless space and strengthening the external borders in response to the growing number of illegal migrations. In the process of accession, as well as at the time of the implementation of the Agreement, the role of the Croatian border police grew and persisted in the modernization and training of professional staff. Croatian border police in today's challenging times functions in a way worthy of the European Union and protects the external borders from illegal attempts to cross, which is evident from the increased number of migrants detected on its territory.