Potential enlargement of the European Union

The 2003 European Council summit in Thessaloniki set integration of the Western Balkans as a priority of EU expansion. This commitment was made in order to stabilise the region in the wake of the Yugoslav Wars, a series of ethnic wars through the 1990s that led to the breakup of Yugoslavia.

Slovenia, the first former Yugoslav country to obtain independence, joined the EU in 2004, followed by Croatia in 2013.

Albania, Serbia, North Macedonia, and Montenegro are all candidate states, and all of them are in negotiations.[15] Bosnia and Herzegovina has applied to join but is not yet recognised as a candidate while Kosovo, which declared independence in 2008, is not recognised by all EU states or by Serbia.

Serbia and Montenegro could join the European Union in 2025.[16] The European Council endorsed starting negotiations with North Macedonia and Albania on 26 March 2020, and they could join after 2025.[17]

Bulgaria blocked North Macedonia's EU Accession Negotiations.[18]

EU countries in green with Turkey shown in orange

Turkey's candidacy to join the EU has been a matter of major significance and considerable controversy since it was granted in 1999. Turkey has had historically close ties with the EU, having an association agreement since 1964,[19] being in a customs union with the EU since 1995 and initially applying to join in 1987. Only after a summit in Brussels on 17 December 2004 (following the major 2004 enlargement), the European Council announced that membership negotiations with Turkey were officially opened on 3 October 2005.

Turkey is the thirteenth largest economy in the world, and is a key regional power.[20][21] In 2006, Carl Bildt, Swedish foreign minister, stated that "[The accession of Turkey] would give the EU a decisive role for stability in the Eastern part of the Mediterranean and the Black Sea, which is clearly in the strategic interest of Europe."[22]

However, others, such as former French President Nicolas Sarkozy and German Chancellor Angela Merkel, maintain opposition to Turkey's membership. Opponents argue that Turkey does not respect the key principles that are expected in a liberal democracy, such as the freedom of expression.[23] Turkey's large population would also alter the balance of power in the representative European institutions. Upon joining the EU, Turkey's 84 million inhabitants would bestow it the largest number of MEPs in the European Parliament.[24] It would become the most populous country in the EU. Another problem is that Turkey does not recognise one EU state, Cyprus, because of the Cyprus dispute and the Cypriot government blocks some chapters of Turkey's talks.

Turkey's relations with the EU have seriously deteriorated in the aftermath of the 2016 Turkish coup d'état attempt and subsequent purges.[25][26] On 24 November 2016, the European Parliament approved a non-binding resolution calling for the "temporary freeze of the ongoing accession negotiations with Turkey" over human rights and rule of law concerns.[27][28][29] On 13 December, the European Council (comprising the heads of state or government of the member states) resolved that it would open no new areas in Turkey's membership talks in the "prevailing circumstances",[30] as Turkey's path toward autocratic rule made progress on EU accession impossible.[31] As of 2017, and especially following Erdoğan's victory in the constitutional referendum, Turkish accession talks are effectively at a standstill.[4][32][33]

The Scottish Independence Referendum of 2014 was the first occasion the EU was faced with the potential breakup of a member state, and one where a newly independent state wished to retain its EU membership. While the UK's withdrawal from the EU also took Scotland out of the EU, the debates in the referendum campaign may inform other future scenarios.

The UK Government's legal advice on the issue was that 'Since the [remainder of the UK] would be the same state as the UK, its EU membership would continue',[154] while speculating that 'On the face of it, Scotland would be required to accede to the EU as a new state, which would require negotiations on the terms of its membership ...', but that 'Scotland's position within the EU is likely to be shaped more by any agreements between the parties than by pre-existing principles of EU law.'[155] Without any formal process for handling the breakup of any member state, the European Commission offered, if requested by a member state, to provide an official view on the EU's position on Scottish EU membership in the event of its independence from the UK. The Scottish Government requested that UK Prime Minister David Cameron place this request, but such a request was not made.[156] Nicola Sturgeon, the then Deputy First Minister of Scotland, said that the Scottish Cabinet did not agree an independent Scotland would have to reapply for EU membership.[147]

The referendum campaigns had differing views:

• Yes Scotland: The "Yes" campaign, led by Blair Jenkins, argued that Scotland would continue as a member state following a Yes vote as Scotland would remain compliant with all EU Principles as outlined in TEU Article 2 and there are no provisions to exclude a state in the existing EU agreements.[157] During the period between a Yes vote and formal independence, the Scottish Government could engage in negotiations, from within the EU, on the terms of their continuing membership in the EU. Several EU heads of state expressed their opinion that this position was reasonable, as did James Crawford, co-author of the UK government's legal advice on the issue.[158] In an interview on BBC Radio, asked if the timescale of 18 months for EU and other treaty organisation was possible, Crawford replied that he felt the timescale was reasonable.[159] However, there was no official comment on this view from the EU Commission. The Scottish Government and the Yes Campaign both declared that continuation of membership in the EU is their preference.
• Better Together: The "No" campaign, led by Alistair Darling, argued that any vote for independence would have automatically placed Scotland out of the EU as a new state, and Scotland would have had to renegotiate entry.[160]

On 1 October 2017, the Catalan government held a referendum on independence, which had been declared illegal by the Constitutional Court of Spain, with potential polling stations being cordoned off by riot police. The subsequent events constituted a political crisis for Catalonia. The EU's position is to keep distance from the crisis while supporting Spain's territorial integrity and constitution.[161][162] While the debate around Scotland's referendum may inform the Catalan crisis, Catalonia is in a distinct situation from Scotland whereby the central government does not recognise the legitimacy of any independence declaration from Catalonia. If Spain does not recognise the independence of a Catalan state, Catalonia cannot separately join the EU and it is still recognised as part of Spain's EU membership.

There is an active movement towards Flemish independence, or union with the Netherlands, with the future status of Wallonia and Brussels (the de facto capital of the EU) unclear as viable political states, perhaps producing a unique situation from Scotland and Catalonia. There are various proposals for what should happen to the city, ranging from staying part of the Belgian rump state, to joining the hypothetical Flemish state, to becoming a separate political entity.[163][164]

Area shown in orange under control of Northern Cyprus

Officially, the island nation of Cyprus is part of the European Union, under the de jure sovereignty of the Republic of Cyprus. Turkish Cypriots are citizens of the Republic of Cyprus and thus of the European Union, and were entitled to vote in the 2004 European Parliament election (though only a few hundred registered). The EU's acquis communautaire is suspended indefinitely in the northern third of the island, which has remained outside the control of the Republic of Cyprus since the Turkish invasion of 1974. The Greek Cypriot community rejected the Annan Plan for the settlement of the Cyprus dispute in a referendum on 24 April 2004. Had the referendum been in favour of the settlement proposal, the island (excluding the British Sovereign Base Areas) would have joined the European Union as the United Cyprus Republic. The European Union's relations with the Turkish Cypriot Community are handled by the European Commission's Directorate-General for Enlargement.[165]

The European Council has recognized that following the UK withdrawal from the EU, if Northern Ireland were to be incorporated into the Republic of Ireland to form a united Ireland it would automatically rejoin the EU under Ireland's membership. This is consistent with the incorporation of East Germany into the Federal Republic of Germany as a single European Communities member state.[166][167] It has also been proposed that an independent Scotland could automatically rejoin the EU by forming a union with the Republic of Ireland.[168]

A similar scenario has been envisioned with the unification of Romania and Moldova, which would incorporate the current territory of Moldova into Romania and thus into the EU.[169]

The Faroe Islands, a self-governing nation within the Kingdom of Denmark, is not part of the EU, as explicitly asserted by both Rome treaties.[170] The relations with the EU are governed by a Fisheries Agreement (1977) and a Free Trade Agreement (1991, revised 1998). The main reason for remaining outside the EU is disagreements about the Common Fisheries Policy,[171] which disfavours countries with large fish resources. Also, every member has to pay for the Common Agricultural Policy, which favours countries having much agriculture which the Faroe Islands does not. When Iceland was in membership negotiations around 2010, there was a hope of better conditions for fish-rich countries, but this came to nothing. The Common Fisheries Policy was introduced in 1970 for the very reason of getting access for the first EC members into waters of candidate countries, UK, Ireland and Denmark with the Faroe Islands.

Nevertheless, there are politicians, mainly in the right-wing Union Party (Sambandsflokkurin), led by their chairman Kaj Leo Johannesen, who would like to see the Faroes as a member of the EU. However, the chairman of the left-wing Republic (Tjóðveldi), Høgni Hoydal, has expressed concerns that if the Faroes were to join the EU as is, they might vanish inside the EU, comparing this with the situation of the Shetland Islands and Åland today, and wants the local government to solve the political situation between the Faroes and Denmark first.[172]

Greenland, a self-governing community that is part of the Kingdom of Denmark, entered the EC with Denmark in 1973, because it was not autonomous at that time. After the establishment of Greenland's home rule in 1979 (effective from 1980), it held a second referendum on membership. The result was (mainly because of the Common Fisheries Policy) to leave, so on 1 February 1985, Greenland left the EEC and EURATOM. Its status was changed to that of an Overseas Country.[173][174] Danish nationals residing in Greenland (i.e. all native population) are nonetheless fully European citizens; they are not, however, entitled to vote in European elections.

There has been some speculation as to whether Greenland may consider rejoining the European Union. On 4 January 2007 the Danish daily Jyllands-Posten quoted the former Danish minister for Greenland, Tom Høyem, as saying "I would not be surprised if Greenland again becomes a member of the EU... The EU needs the Arctic window and Greenland cannot alone manage the gigantic Arctic possibilities".[175] Greenland has a lot of natural resources, and Greenland has, especially during the 2000s commodities boom, contracted private companies to exploit some of them, but the cost is considered too high, as Greenland is remote and severely lacks infrastructure. After 2013 prices have declined so such efforts have stalled.

The Brexit debate has reignited talk about the EU in Greenland with calls for the island to join the Union again.[176]

The islands of Aruba, Curaçao, and Sint Maarten are constituent countries of the Kingdom of the Netherlands, while Bonaire, Sint Eustatius and Saba are special Dutch municipalities. All are Overseas Countries and Territories (OCT) under Annex II of the EC treaty.[173] OCTs are considered to be "associated" with the EU and apply some portions of EU law. The islands are opting to become an Outermost Region (OMR) of the EU, a status in which the islands form a part of the European Union, though they benefit from derogations (exceptions) from some EU laws due to their geographical remoteness from mainland Europe. The islands are focusing on gaining the same status as the Azores, Madeira, the Canary Islands and the French overseas departments.

When Bonaire, Sint Eustatius, and Saba were established as Dutch public bodies after the dissolution of the Netherlands Antilles (which was an OCT) in 2010, their status within the EU was raised. Rather than change their status from an OCT to an outermost region, as their change in status within the Netherlands would imply, it was decided that their status would remain the same for at least five years. After those five years, their status would be reviewed.

If it was decided that one or all of the islands wish to integrate more with the EU then the Treaty of Lisbon provides for that following a unanimous decision from the European Council.[177] Former European Commissioner for Enlargement Danuta Hübner has said before the European Parliament that she does not expect many problems to occur with such a status change, as the population of the islands is only a few thousand people.

The territories of French Guiana, Guadeloupe, Martinique, Mayotte and Réunion are overseas departments of France and at the same time mono-departmental overseas regions. According to the EC treaty (article 299 2), all of these departments are outermost regions (OMR) of the EU—hence provisions of the EC treaty apply there while derogations are allowed. The status of the Overseas collectivity of Saint-Martin is also defined as OMR by the Treaty of Lisbon. New Caledonia and the overseas collectivities of French Polynesia, Saint-Barthelemy, Saint Pierre and Miquelon as well as Wallis and Futuna are Overseas Countries and Territories of the EU.[173]

New Caledonia is an overseas territory of France with its own unique status under the French Constitution, which is distinct from that of overseas departments and collectivities. It is defined as an "overseas country" under the 1998 Nouméa Accord, and enjoys a high degree of self-government.[178] Currently, in regard to the EU, it is one of the Overseas Countries and Territories (OCT).

As a result of the Nouméa Accord, New Caledonians voted on an independence referendum in November 2018. This referendum was to determine whether the territory would remain a part of the French Republic as a "sui generis collectivity", or whether it would become an independent state. The accords also specify a gradual devolution of powers to the local New Caledonian assembly. The referendum's result was that New Caledonia would remain a part of the French Republic.