In what year will Montenegro become a full member of the European Union (i.e., in what year will its EU accession treaty enter into force)?

rationale:

Despite Montenegro remaining the clear frontrunner with all 33 chapters open and 18 provisionally closed as of mid-2026 3 sources, accession before 2028 is effectively impossible (0%). The resolution bar is the entry into force of the accession treaty, which entails a grueling procedural timeline. After all chapters close, a treaty must be drafted—a process that took roughly 10 months for Croatia europeanwesternbalkans.com—receive European Parliament consent, unanimous Council approval, and signature. Finally, it must be ratified by Montenegro and all 27 (or more) EU member states, which historically takes 13 to 21 months post-signature europeanwesternbalkans.com. While the political target of "28 by '28" exists apnews.com, achieving a 2028 entry (8%) would require flawless execution: finalizing the 15 remaining chapters immediately, signing the treaty by mid-2027, and moving through 27 national parliaments at record speed. Tellingly, the European Commission's own financial packages are explicitly built to accommodate accession after January 1, 2028 ec.europa.eu.

The Core Window (2029–2031) The bulk of the probability mass falls between 2029 and 2031, with 2029 (23%) and 2030 (22%) acting as the most realistic modal years. A realistic schedule anticipates chapter closure slipping into 2027, driven by the complex final benchmarks of the rule-of-law chapters (23 and 24), which govern the judiciary, fundamental rights, and anti-corruption metrics 2 sources. If the accession treaty is signed in late 2027 or 2028, a standard 1.5 to 2.5 year ratification period squarely places full accession around 2029 or 2030. This aligns with informal working horizons within the Commission targeting a small group of new members "around 2030" carnegieendowment.org.

Vetoes and Ratification Risks A significant share of the uncertainty lies in the ratification phase, which is highly exposed to national-level vetoes and domestic politics. Croatia currently leverages its veto over bilateral disputes—such as maritime boundaries, war-crimes prosecutions, and the training ship Jadran—blocking chapters like 14 and 31 2 sources. Furthermore, French constitutional law (Article 88-5) generally requires a referendum for EU accession treaties unless bypassed by a three-fifths majority in both parliamentary chambers institutdelors.eu. In a fragmented French political landscape with high public skepticism regarding enlargement, this presents a formidable hurdle that could heavily delay the timeline.

Broader EU Reforms and the Long Tail Finally, the 11% probability for accession occurring "after 2035 or never" reflects the risk of structural gridlock or indefinite delay. France, Germany, and the Netherlands are pressing for deep institutional reforms and stringent safeguard clauses to prevent a "Hungary 2.0" scenario before admitting new members 3 sources. If EU internal reform becomes a strict precondition for enlargement, or if Montenegro suffers domestic political instability and rule-of-law backsliding, the process could stall indefinitely. Historically, every previous Montenegrin target date (2025, 2026, 2028) has slipped, justifying a cautious outlook on long-term downside risks.