This paper reports on a case study on the EU’s enlargement process which aims to provide an empirical answer to the question whether the European integration is irreversible or not. In line with a Habermasian logic, the study has found that both the existing members’ and new comers’ behavioural patterns were mainly shaped by instrumental/strategic actions, which were inappropriate to the logic of appropriateness, during the enlargement process from the 1970s to the 2010s. Therefore, this finding supports the argument that the European integration is an outcome of member states’ deliberative actions; thus, it might be reversible when member states’ significant interests clash with this integration process. However, to provide more comprehensive explanations about this question, it might be a better idea to visit ‘agent-structure problem’ in the discipline of IR.