The European Union has not enlarged for the longest period in its history since the first enlargement in 1973. In recent years, however, talk of ‘enlargement fatigue’ has been replaced by an emphasis on the geostrategic importance of enlargement, primarily in response to the Russian invasion of and war in Ukraine. This begs the question of whether at least some of the current nine candidates will accede to the Union in the foreseeable future. While the prospects for enlargement rely on candidates making meaningful progress in meeting the requirements to join the Union, they also require the European Union to agree to enlargement. We consider five key variables to assess the state of play with European Union enlargement: the Union commitment, Member State preferences, supranational activism, integration capacity, and public opinion.

The Windsor Framework establishes a unique status for Northern Ireland in the UK‐EU relationship that has proven controversial, particularly for unionists. It also provides members of the Northern Ireland Assembly with regular opportunities to give their ‘democratic consent’ to the continued application of arrangements in the Windsor Framework that place Northern Ireland through regulatory alignment de facto in the EU's customs territory and internal market for goods. A first vote in December 2024 saw a majority in favour of consent, but with all unionist MLAs voting against. The absence of a cross‐community majority for consent led to the UK government establishing an independent review of the operation of the Windsor Framework, charged with making practical recommendations that command cross‐community support. A closer UK relationship with the EU could address some concerns. The Northern Ireland Assembly has a further opportunity to give its consent in 2028.

Withdrawal from the European Union (EU) has seen the United Kingdom (UK) abandon membership of and formal influence over policy making in the EU and pursue a policy of disintegration from and minimal commitments towards the EU. Although the UK-EU Trade and Cooperation Agreement is strikingly lacking in ambition, it does involve a dense and complex institutional framework to manage relations. Moreover, the post-Brexit UK-EU relationship includes a unique set of arrangements that keep Northern Ireland in the EU's customs territory and internal market for goods. These arrangements involve dynamic regulatory alignment and are managed by a dedicated institutional set-up that has the potential at least to provide the UK 'in respect of Northern Ireland' with a particular decision-shaping role with regard to applicable areas of the EU acquis. This article considers the initial operation of these sets of institutional frameworks and their potential for UK influence over the EU.

This introductory article makes the case for studying joint institutional frameworks (JIFs) in EU bilateral agreements and provides an overview of the remaining contributions to the symposium. In doing so, it addresses contemporary policy developments and theoretical debates in political science and international institutional law. It considers the rationale, design, performance as well as legitimacy of JIFs both in general and, in particular, in the EU's contractual bilateral relations. By mapping out the variety of JIFs in distinct geographical and regulatory contexts, the article develops an overarching argument about the ‘transversal’ nature of such structural frameworks, focusing on the most prevalent structural principles and rules, joint bodies and special procedures, including those not covered in detail in the other contributions to this symposium.

This chapter explores how the EU ended a long period of constitutional change by agreeing the Treaty of Lisbon and used it to face new challenges of financial crisis, Brexit, and Covid-19—the latter events leading to thoughts that further treaty change might be needed. The process started with the 2002–03 Convention on the Future of Europe leadin to the Constitutional Treaty of 2004 and in October 2007 produced the Treaty of Lisbon which eventually entered into force on 1 December 2009. Its implementation was complicated by the eurozone crisis, resulting in extra-treaty arrangements and another treaty amendment. Although the official appetite for treaty reform all but evaporated in the 2010s, the UK’s June 2016 vote to quit the EU raised the hopes for further changes. The end of the 2010s and into the 2020s saw Brexit being negotiated within the terms of the Treaty of European Union the EU’s treaty agreeing measures to deal with the Covid-19 pandemic. Calls for treaty revision continued but active steps to re-negotiate the consolidated treaties have not yet begun.

Brexit and the United Kingdom’s withdrawal from the European Union (EU) customs union and single market poses a multitude of economic and political as well as other challenges for Northern Ireland, much of which was encapsulated in the shared UK and EU commitments during the withdrawal negotiations to avoid a hard border on the island of Ireland and uphold the 1998 Belfast (‘Good Friday’) Agreement. Awareness of the challenges led to calls for ‘flexible and imaginative solutions’ and therefore the possibility at least of some form of differentiated withdrawal for Northern Ireland. This chapter considers the options that were proposed, the actual terms of withdrawal and some of the consequences of these for Northern Ireland, the island of Ireland, the United Kingdom and the EU. In doing so, it notes the establishment of a new dimension to the study of differentiated integration, namely the differentiated treatment of a territorially and constitutionally specific region of a non-member state.

This article takes a comparative perspective to identify the likely parameters of the future partnership between the United Kingdom (UK) and the European Union (EU). It asks why and how the EU exports institutional norms to its neighbouring countries, and what this implies for the EU-UK relationship. Drawing on historical institutionalism, the authors argue that as a specific legal-institutional order, the EU exports not only regulatory norms and values but also institutional norms into its partnerships with neighbouring countries, mainly through the mechanism of reproduction. For the UK this means that the nature of its future partnership with the EU is likely to be influenced more by established practices in and precedents from the EU's relations with European neighbours than by any sense of privilege emanating from the UK's position as a former member state.

The focus of this chapter is the emergence of the European Communities in the 1950s, their evolution in the three decades thereafter, and the establishment and early development of the European Union (EU) in the 1990s. The chapter explores key developments in the first four decades of European integration and some of the tensions that have shaped them. It considers the ambitions of the architects and supporters of the European Communities and how their hopes and aspirations played out as integration became a reality in the 1950s and 1960s. It looks at how their ambitions grew and how the process then lost momentum in the 1970s before the idea of ‘European union’ was rekindled in the 1980s with the Single European Act (1986) and the Single Market project. These acted as catalysts for a new era of dynamic European integration with the now expanded Communities at its core. The chapter then explores how, through ‘Maastricht’ and the adoption and implementation of the Treaty on European Union (1992), the European Union was established. The chapter assesses the unique and incomplete form of the new ‘union’ and examines the impact on it of reforms introduced by the Treaty of Amsterdam (1997) and the Treaty of Nice (2000) as the EU sought to prepare itself for the further enlargement and the challenges of the initial years of the twenty-first century.