Then in the small hours came a political thunderbolt almost no one had forecast: the UK had voted to leave the union. Five years on, the Brexit tempest has subsided – in Brussels, if not in London.
“Not only did the EU survive the storm, but it also moved on,” said Georg Riekeles, of the European Policy Centre thinktank, and formerly an adviser to the EU’s Brexit negotiator, Michel Barnier.
Referring to the agenda of Marine Le Pen, Geert Wilders and other leaders of far-right anti-EU parties, Mr Riekeles said that ahead of the Brexit vote “there was very clearly a populist, disintegrationist drive, ‘let’s break it up’”.
“Now,” said Bernd Lange, a German social democrat MEP, “even the rightwing populists aren’t discussing leaving the European Union, the Frexit [threat of French exit] is gone. They are saying we need to change the European Union.”
The EU is still hobbled by deep splits, whether over helping refugees or fixing flaws in the currency union. Many officials see the deepest threat emanating from Hungary and Poland, where powerful ruling parties are weakening independent courts and media, widely seen as an internal “exit” from the rule of law at the heart of Europe.
Hungary’s prime minister, Viktor Orbán, provoked fresh outrage this week when he proposed allowing national parliaments to suspend the EU legislative process and called for the term “ever closer union” to be deleted from the union treaty.
Against this backdrop, Brexit helped the EU rediscover its raison d’être, said Mr Riekeles. “The EU as a whole … even the capitals of countries that on topics like rule of law are outliers or very difficult members of the family, showed the capacity to rally together around an existential issue.”
Smaller EU member states were impressed by how the EU has stood behind Ireland in the disputes with the UK over the border, which had raised concern about the peace process, he said.
Other divisions did not ricochet into Brexit talks. Instead, the process forced the 27 member states to pay more attention to each other, said Nathalie Loiseau, an MEP, who was France’s Europe minister.
Ms Loiseau added: “It created more common culture of what it means to have a single market, to have a level playing field, and this will play a role in the future in our relations with the rest of the world.”
During the negotiations, even seasoned diplomats admitted the Brexit process had taught them new things about the EU single market and customs union. The deepened appreciation of the EU’s building blocks was forged not only by Brexit, but the election of a US president that same year who wanted to tear up the multilateral system and who declared the EU a foe.
“Defending the internal market I think translates into a more proactive and more assertive European Union in other economic areas,” said Rem Korteweg, a senior research fellow at the Clingendael institute, in the Netherlands.
That has continued even after Donald Trump was replaced by a friendlier US presidential figure, Joe Biden. EU leaders ignored concerns of the incoming Biden administration when they signed a trade deal with China last December, although the pact looks unlikely to come into force.
The EU seeks a less confrontational approach to Beijing, in contrast to the US. “The EU is very assertive and even self-confident in saying look we have our own story to tell in saying how global trade should develop,” Mr Korteweg said.
While many mourned the British departure, Brexit made it easier for the EU to make progress on long-stalled policies. Soon after the Brexit vote, the EU activated common defence plans that had lain on the drawing board for years.
Although Ms Loiseau said she thought the British would have been putting the brakes on these plans, she said she missed the UK. “We miss this world vision, which the UK obviously had,” she said. The past tense expressed scepticism about the UK’s “global Britain” aspirations, a view Boris Johnson’s government would contest.
The coronavirus pandemic also led to an economic step once thought politically impossible: joint borrowing from financial markets to fund an €800bn recovery fund. For some, the EU recovery fund underscored the power of France and Germany, the two governments that forged the plan and then put it to the rest.
“It was always an unspoken truth that Germany and France were primus inter pares, the primary players, but I think Brexit has accentuated that,” said Mr Korteweg. “It raises questions particularly for the northern European member states that tended to gravitate towards London on particular issues.”
In response, the Dutch government had sought to work more closely with France. “What we see the Dutch doing is actually bandwagoning, saying OK, if you can’t beat them join them.”
In the parallel universe where Remain won a narrow victory in 2016, it is hard to imagine any British prime minister being able to sell shared debt without strong political headwinds.
“Would [the EU recovery fund] have happened the way it did? Probably not,” Mr Koretweg said. “I think there would have been tremendous pushback in London and probably so strong that it would either have been watered down or not have happened at all.”
Given the British role as leader of the EU’s awkward squad, obtaining opt-outs and raising red flags, some things are easier without the UK. “There are different states of sorrow,” said Mr Riekeles. “We miss the British, but probably less than we thought.”