“My red line is the unity of the EU27”, Barnier told prime ministers in 2016 at their first encounter. Towards the end of 2017, commentators, journalists and experts asked members of Barnier's team more frequently when this surprising unity would crack and dissolve. After so many years of divisions between member states, people assumed that EU unity was unnatural and unsustainable. The most important element that explains the unity was the sense of political responsibility of government leaders who thought that a divided reaction could endanger European integration. The unity grew in those first months since June 2016 on both the negotiation approach and substance. Institutionally, Tusk decided that Brexit was a matter for the 27 only, never to be discussed collectively with the UK prime minister. The preparatory work of Juncker, Barnier and his team, often working hand-in-hand with Council and national officials, resulted in a remarkably clear negotiation mandate for Barnier in May 2017.

EU decisions are often fragile compromises that can pay a price of ambiguity and complexity to overcome national divisions. In Brexit, unity led to clarity, which in turn sustained unity. That clarity reaffirmed founding principles on fundamental freedoms, the single market and the autonomy of the EU's legal and political order. Once adopted, it became hard for any member state to move away from them. Moreover, the principles created a take-itor-leave it situation for the UK on its withdrawal terms. In other words, it succeeded in putting the burden of sorting out the meaning of Brexit solely on the UK, which is what the EU wanted and ran contrary to what some UK politicians had expected.

The EU's unity was not just about politics. Collective policy learning based on Barnier's inclusive method of work was important. The first seminar on citizens’ rights between the Commission, national diplomats and MEPs in 2016 was the start of a search towards a consolidated EU position. Some national experts thought a mixed EU-national agreement was in order, as residency rights and social security are national competences.