In the period from 2001 to 2006, the Western Balkans served as a testing ground for the development of the EU’s Common Foreign and Security Policy. This paper explores a variety of legal and political aspects of the EU’s efforts to stabilise the Western Balkans in an attempt to answer the question about what the EU should have learned in so doing. Attention is paid to the diplomatic efforts of the EU to prevent (the escalation of) conflict in Macedonia, between Serbia and Montenegro, and in Kosovo. The role and impact of the EU’s first-ever – and so far biggest – police and military operations are also evaluated. It is argued that lessons learned from these actions should be taken to heart before the EU decides to embark on any future missions. Otherwise, history may prove that the Western Balkans offered the EU’s one and only chance to develop credible and lasting foreign policy, security, and defence arrangements.