This thesis looks into how bilateral issues feed into ongoing EU accession negotiations and are, in fact, able to stall the process. Further, this piece of research analyses whether and how it is possible to resolve bilateral issues during accession negotiations. The salience of this single-case study of the border dispute between Croatia and Slovenia is the hitherto non-existent situation of two EU Member States facing an unresolved bilateral conflict over territory, here a so-called mixed dispute (maritime and land). The study reconstructs the conflict and both its bilateral and third-party management efforts between the end of Yugoslavia and the most recent stage, the lawsuit before the CJEU, with focus on the events 2008-2018 including the troublesome arbitration procedure from 2012-2017. The method employed in this qualitative within-case study is process tracing. The availability of both (i) actors for interviews and (ii) documents renders process tracing particularly apt. Interviews play a central role and are at the heart of this study. Actors are vital sources of information and can help reconstruct events and the causal mechanisms between them. Documents are the second pillar of the data collection approach this study is based on. Draft documents in particular are a first-hand primary source and an equally crucial instrument alongside interviews in the process tracer’s toolbox. Prior to Croatia’s EU accession on 01 July 2013, this very conflict had led to a blockade of the accession negotiations of one party to the conflict (Croatia) by the other party to the conflict that was already an EU member (Slovenia). The inherent feature of the present case is that an issue of national interest has been pursued by means of the Member State veto power. That leverage is indeed at hand not least when bilateral attempts have failed. As a result, the asymmetric power relationship between the two countries at the time produced some coercive momentum to ‘solve’ the conflict, or to initiate such solving, during Croatia’s accession negotiations. These features are unique in the history of EU enlargement. In view of the implications for the post-Yugoslav space in relation to the EU, an analysis of how a bilateral conflict between an EU Member State and a Candidate Country was handled is urgently needed. That is what this study will want to provide. The recent so-called Western Balkans Strategies of the European Commission from 2012 and 2018 confirmed the merits-based approach meaning each Candidate Country is going to join the EU when it fully complies with EU conditionality individually and irrespective of the progress of other Candidate Countries. It follows from the current merits-based approach that it appears highly unlikely that the remaining SFRY successor States and Albania will be able to join at the same time. As a result, every time one Candidate Country has joined, it acquires the Member State veto power vis-à-vis another Candidate Country, so conflict over bilateral issues between Member States and Candidate Countries may become self-energising. This study presents concluding recommendations with regard to, first, a sustainable implementation of the common State border between Croatia and Slovenia; second, pressing bilateral issues in the ongoing and prospective accession negotiations with the Candidate Countries from the former Yugoslavia; third, means to strengthen dispute resolution both in terms of an EU framework and concerning reform of international arbitration; and fourth, ways to create some responsiveness between the rule-of-law record of and financial support for Candidate Countries.