Croatia is blocking Montenegro from closing an EU accession chapter that the bloc's other 26 member states are prepared to close, the latest in a series of long-running bilateral disputes that have turned into a wider test of how far existing EU members can use the enlargement process to exert pressure on aspiring members.

The dispute is not an isolated feature of Montenegro's negotiations. EU member states have repeatedly used their ability to veto decisions in the accession process to press candidate countries over bilateral disputes, from Bulgarian objections relating to North Macedonia's identity and language to Cyprus's blocking of Turkey and Slovenia's use of the process against Croatia.

In Montenegro's case, Croatia has withheld support for closing the chapter covering EU transport rules and has also blocked progress on foreign policy and judiciary-related areas. At an intergovernmental conference in July, Zagreb prevented Montenegro from closing Chapter 31, covering foreign, security and defence policy.

Croatia says its position is justified by the EU's requirement for good-neighbourly relations. In a July 22 foreign ministry statement, it listed compensation for former detainees of wartime camps, locating 14 people still missing from the 1990s wars, war-crimes prosecutions, property claims by Croatian families in Montenegro and related stalled court cases among the issues it wants resolved.

It also called for the preservation of a memorial plaque at the site of the former Morinj detention camp, continued talks on the maritime boundary, the return of the training ship Jadran — and the renaming of a swimming pool in the coastal city of Kotor. All have become entangled, to varying degrees, with the process by which a country of around 620,000 people is seeking to join the union of 27 states.

Some of the issues Zagreb has raised are serious matters of justice and historical accountability. Locating the 14 people still missing from the wars of the 1990s, resolving wartime detention claims and pursuing war-crimes cases are not routine accession conditions.

Tensions resurfaced in Back in 2024, an immediate deterioration in relations followed a resolution on the Jasenovac concentration camp adopted by Montenegro's parliament. Jasenovac was a concentration camp operated by the wartime Independent State of Croatia, where tens of thousands of people were killed during World War Two. Croatian Prime Minister Andrej Plenković complained that Montenegro's resolution was "unacceptable, inappropriate, and unnecessary”. Another past wound resurfaced when, Mandić sent condolences to the family of convicted Bosnian Serb war criminal Ratko Mladić, further straining relations.

But the fact that such questions can be placed alongside a demand to rename a swimming pool illustrates the central problem. Under the EU's current system, an individual member state can connect its bilateral agenda to a candidate's progress towards membership even when the other 26 governments are willing to move ahead.

That gives existing members a powerful instrument to pursue disputes that may have little direct connection with the chapter being considered.

The European Commission has sought to keep the disputes from derailing Montenegro's wider accession bid. Enlargement Commissioner Marta Kos urged Croatia in August to resolve its bilateral issues with Montenegro, although the Commission has limited leverage over a member state that chooses to withhold its consent, as previous disputes between existing and aspiring members have shown.

Montenegro has opened all 33 negotiating chapters and, according to the European Commission, had provisionally closed 16 by the end of the latest reporting period. The country wants to complete negotiations by the end of 2026 and join the EU in 2028.

Montenegro's chief negotiator with the EU, Predrag Zenović, said in a September 5 interview with Radio Montenegro that completing the negotiations by the end of this year remained possible and that 2028 membership was a realistic target, provided reforms continued and the country secured the support of all 27 member states.

Zenović played down the risk posed by the dispute with Croatia, pointing to Zagreb's support for closing 14 other chapters and arguing that this demonstrated that dialogue remained possible. He also stressed that geopolitical circumstances could accelerate enlargement but could not substitute for reforms, particularly in the rule of law.

Zagreb has direct experience of the same mechanism. Slovenia blocked Croatia's accession process in 2009 over a dispute concerning the maritime boundary in the Bay of Piran. The dispute eventually went to arbitration, which ruled in Slovenia's favour in 2017, although Croatia rejected the ruling. In 2020, the European Court of Justice said it did not have jurisdiction over the bilateral dispute.

Croatia later used its own leverage against Serbia, blocking negotiations on chapters 23 and 24 for several months before the objections were lifted.

Other EU members have followed similar tactics. Bulgaria blocked North Macedonia's accession process in 2020 over disputes involving history, language and national identity, after Greece had for years opposed the country's previous name, Macedonia, in a separate dispute. The two disputes have contributed to North Macedonia's unusually long wait after it became an EU candidate back in 200xxxx.

Cyprus has used the accession process against Turkey, blocking its transport chapter in 2007 and subsequently preventing the opening of several other chapters. The parallel with Montenegro is particularly close: the transport chapter now being held up by Croatia is the same negotiating area Cyprus blocked for Turkey nearly two decades ago.

Hungary has also blocked the opening of additional negotiating clusters for Ukraine this month, with the position continuing despite a change of government in Budapest.

The recurrence of such disputes has prompted repeated efforts to reform the EU enlargement process without changing the fundamental requirement for unanimity among member states.

The European Commission revised its enlargement methodology in 2020, grouping negotiating chapters into thematic clusters and introducing the possibility of reversing progress if reforms deteriorate. The changes came after French President Emmanuel Macron had opposed opening accession talks with Albania and North Macedonia in 2019.

But the revised methodology did not remove the ability of individual member states to block progress. Bulgaria vetoed North Macedonia the following year.

Germany and Slovenia subsequently proposed a more fundamental change, under which decisions during the accession process would in many cases be taken by qualified majority rather than unanimity. Sixteen of the EU's 27 members backed the idea. Austria said it would advocate qualified-majority voting for accession decisions.

At an informal European Council meeting in Copenhagen in October 2025, Council President António Costa put the proposal forward, but it was rejected. Hungary's then prime minister Viktor Orbán was among its strongest opponents, while France, the Netherlands and Greece also opposed it.

The debate has consequently shifted towards finding ways to reduce the consequences of national vetoes rather than removing them altogether. The European Commission has also examined concepts involving partial or reversible integration, allowing candidate countries to participate progressively in parts of the single market and other EU structures.

The debate on both enlargement and the EU’s decision-making processes has come to the fore recently, with officials acknowledging that changes are most likely needed before the bloc can embark on a new wave of accession to the Western Balkans and Eastern Europe similar in scale to the 2004 expansion. Kos said on August 25 that internal reform without treaty change "could also mean reducing the number of areas where a single member state can block everyone else".

At the same time, she cautioned against weakening safeguards for the union itself. "We must make sure we do not let in Trojan horses. Not today, not in 10 or 20 years from now," Kos said. The Commission is due to put forward more detailed proposals ahead of EU leaders' discussions on enlargement in October.

Another approach would be to separate bilateral disputes from accession altogether. Tefta Kelmendi of the European Council on Foreign Relations (ECFR) argued in a recent paper that bilateral conflicts should be dealt with through parallel European institutions and processes rather than being allowed to determine whether a candidate advances towards membership. She has suggested using the European Political Community and Council of Europe to address such disputes and extending the EU's Growth Plan benefits to candidate countries regardless of unresolved bilateral issues.

For accession candidates, some of which have spent well over a decade in negotiations, the distinction between reforming to meet EU standards and negotiating with individual EU capitals has become increasingly difficult to maintain. Until the EU changes the rules, or member states agree to use their vetoes differently, that distinction will remain in the hands of the countries already inside the club.