This briefing explores the motives, narratives, and membership realities of the three states that acceded to the European Communities (EC) in its first enlargement, in 1973: the United Kingdom, Ireland, and Denmark. It argues that, despite the different conditions and context at the time, it is possible to draw lessons from this first enlargement experience on how applicant states can create favourable conditions for their stable long-term integration into the present-day European Union.Governments, political parties and societal groups need to define their expectations of the economic benefits of EU membership realistically. This was the case in Ireland and Denmark, which rightly expected to benefit from the common agricultural policy and – in the case of Ireland – significant fiscal transfers. It was not the case for the UK, however, leading to a negative collective experience of the economic realities of membership during the economic slump following the 1973 oil crisis. Politically, moreover, the UK was ill prepared for cooperative patterns of politics and policy- making in the EC/EU. Large sections of the British elites and citizens saw the world in a binary divide between those ruling and those being ruled. EC membership could easily be portrayed as mediating the UK's transition from a position of ruling to one of being ruled by an allegedly new imperial centre, 'Brussels'. By contrast, EC membership gave the Irish an institutionalised European voice. Sharing formal sovereignty for them enhanced their actual independence from the UK. Denmark, finally, over time developed a pragmatic approach to sharing sovereignty.This briefing also highlights the importance of defending the aim of EU membership with realistic and convincing narratives. The creation of a broad domestic consensus on the desirability of membership, and preparation for its economic and political implications, are crucial for shaping a trajectory towards stable long-term integration into the EU.This briefing follows up a roundtable event organised by EPRS on 26 April 2023, which included contributions by N. Piers Ludlow, London School of Economics, Brigid Laffan, European University Institute, and Morten Rasmussen, Copenhagen University.