On 24 June 2016, the day after Britain voted to leave the European Union, Jean-Claude Juncker, Donald Tusk and Martin Schulz issued the EU’s first formal response to a decision they described in a joint statement as “regrettable”, but said they respected.

The bloc stood united, the presidents of the European commission, council and parliament said. Defence of its stability and interests was its priority, so any Brexit agreement with the UK would be “balanced … in terms of rights and obligations”.

In the days that followed, Angela Merkel, the German chancellor, and François Hollande, then France’s president, put meat on the bones of that anodyne little phrase. “There can be,” Merkel told the Bundestag – adopting a term destined to become famous – “no cherry-picking.” There must, she stressed, be “a palpable difference between countries who are members of the European family, and those who are not.”

Hollande was even plainer: an “à la carte” EU was not on the menu, he said. Being in the single market “has advantages. The UK must face the consequences of its decision”.

Two-and-a-half years on, the withdrawal agreement and political declaration are all but complete, and the first phase of the process is finally over – at least for the EU27.

A lot on the continent has changed since. Regret remains, but a page has turned.

Schulz has sunk without trace in Germany; Hollande is no longer president of France; Merkel will not stand again. Populist, far-right and Eurosceptic parties have risen (or returned) to power in Italy, Austria and Hungary and made record gains in Sweden.

However, in a famously fractious bloc which is often unable to agree on anything much, the common understanding on Brexit reached, very rapidly, by 27 different capitals in early summer 2016 has not just survived but intensified into an unlikely unity of purpose that, ultimately, made the divorce talks’ outcome inevitable.

Each individual EU member state may be facing different consequences and costs from Brexit, and each – as well as the bloc’s central institutions – necessarily has a different experience of the talks. But ask for their top Brexit takeaway so far, and the most common response is: EU solidarity matters, and it works.

That, and some surprise at the UK’s ineptitude. “It was clear to most Europeans that the Brexiters’ promises were empty,” said Anna-Lena Högenauer of Luxembourg’s Institute of Political Sciences. “Most, though, thought it was just politics, that there was some kind of plan behind the rhetoric.”

The discovery that there was not, Högenauer said, was something of a shock. “The UK basically jumped into the ocean blindfold, and started paddling round in circles. It was almost like it expected the EU not just to say what Brexit should look like, but to devise a version of it that would suit Britain.”

Angela Merkel set out the position of the EU27. Photograph: Marquardt Christian/Rex/Shutterstock

Europe’s unity was far from a given at the outset, said Fabian Zuleeg of the European Policy Centre, a leading Brussels thinktank. “There was real regret, of course, but also huge uncertainty, in the immediate aftermath of the referendum,” Zuleeg said. “Real concern, if not fear, of what might come next, and of how the process might unfold.

“But it led to a quite remarkable coming together of the EU. The key red lines, and the sequencing, were set very early.”

The EU, uncharacteristically, was moving purposefully on. “It was an existential moment,” said Salvador Llaudes, of the Universidad Autónoma of Madrid. “A moment of truth. There was an understanding that a partner was leaving – not just any partner – and that how the EU responded would decide its future.”

The foundations of Michel Barnier’s mandate as the European Union’s chief negotiator – no negotiation before article 50; no cherry-picking benefits of the single market without accepting all its obligations; no trade talks before Brexit – were laid at an EU summit held less than a week after the referendum. And they have not budged significantly since.

“That political mandate was established fast, and Barnier has executed it faithfully,” said Guntram Wolff of the European thinktank Bruegel.

“He travelled to all the capitals, took on board every view. It’s proved a highly efficient mechanism, one voice speaking for all.”

It helped, Wolff said, that most member states were “only too happy not to have to worry about the day-to-day details of such a complicated and painful procedure. They have a lot of other stuff on their plates. For many, Brexit very quickly became a second-tier, even a third-tier, issue.”

Not for all. The Dutch, who have calculated that even a best-case Brexit could knock 1.2% off their economy by 2030, have been “pragmatic and pro-active” perhaps more than any other country, said Sarah de Lange of the University of Amsterdam.

Quick Guide Why extend the Brexit transition period? Show Will the proposal solve anything? The mooted extension to the transition period is a new idea being put forward by the EU to help Theresa May square the circle created by the written agreement last December and the draft withdrawal agreement in March.

That committed the UK and the EU to ensuring there was no divergence between Northern Ireland and the Republic of Ireland.

But it also, after an intervention by the Democratic Unionist party, committed the UK (not the EU) not to have any trading differences between Northern Ireland and Great Britain.

The problem is that these are two irreconcilable agreements. They also impinge on the legally binding Good Friday agreement, which brought peace to Northern Ireland and in some senses pooled sovereignty of Northern Ireland giving people a birthright to be Irish or British or both.

If the UK leaves the EU along with the customs union and the single market then the border in Ireland becomes the only land border between the UK and the EU forcing customs, tax and regulatory controls. The backstop is one of three options agreed by the EU and the UK in December and would only come into play if option A (overall agreement) or option B (a tailor-made solution) cannot be agreed by the end of transition. The Irish have likened it to an insurance policy. The new EU idea is to extend the transition period to allow time to get to option A or B.

But an extension is problematic for Brexiters and leave voters, who want the UK to get out of the EU as soon as possible.

The Irish and the EU will also still need the backstop in the withdrawal agreement, which must be signed before the business of the trade deal can get under way. Otherwise it is a no-deal Brexit.

Extending the transition into 2021 would mean another year of paying into the EU budget. Britain would have to negotiate this but it has been estimated at anywhere between £10bn and £17bn. Staying in the EU for another year would also mean continued freedom of movement and being under the European court of justice, which Brexiters would oppose.

“We are so dependent on an open economy and trade that we started work almost immediately on minimising Brexit’s impact,” De Lange said. “We’ve put in a lot of hard work, really a lot, to ensure a smooth transition, no matter what the Brexit outcome is. We’re prepared.”

For Germany, said Ulrich Speck of the German Marshall Fund, economic issues, while real, were largely outweighed by broader geopolitical concerns: “Where will Britain stand on the global stage; can it remain an outward-looking country and not be utterly consumed by Brexit?”

But Berlin’s overriding priority, Speck said, was “to protect our common European space. If we open it up, it will unravel.

“Even in countries with governments sometimes highly critical of the EU, what mattered was not to rock the boat. This has not been about punishing Britain, but about protecting the EU.”

And German business, with its formidable car industry, was never going to push Merkel for a soft deal for Britain, as Brexiters fondly imagined, Speck added. “It’s quite simple: German companies care a lot more about the single market than about the British market. Germany’s national interests lie with the EU.”

France, too, has commercial interests to defend, said Christian Lequesne of SciencesPo Paris, including transport and fisheries: more than 50% of its Brittany, Normandy and north coast fleets’ catch comes from UK waters. But national interests have largely been subsumed in the “remarkable cohesive effect” of Brexit.

“Britain’s strategic error was to threaten the integrity of the single market” by suggesting that full access and frictionless trade were possible outside it, said Lequesne. “For France and Germany in particular, that’s the EU’s soul.”

However, Brexit has also had an impact on the continent’s Eurosceptics, he added. “It’s striking. Before and right after the referendum, the talk was Frexit, Nexit, Czexit. It all stopped very quickly. In France, Marine Le Pen wanted to leave the EU, then the euro, now she wants to leave neither. No one wants to.”

First, Lequesne said, Britain’s experience “has shown how complicated leaving is. Second, Brexit has boosted popular support for the EU.” October’s Eurobarometer survey shows that 68% of respondents feel their country benefits from EU membership, the highest level since 1983, while 66% – a majority in every state – said they would vote to remain in any exit referendum.

In Poland, said Natasza Styczyńska of Jagiellonian university – co-author of a revealing report by the UK in a Changing EU thinktank on EU attitudes to Brexit – the key concerns were the rights of the 1 million-plus Polish citizens in the UK, and the UK’s EU budget contribution.

Kraków, Poland. The rights of Polish people in the UK are of concern to the country’s leaders. Photograph: Julie Ault/AP

Spain was worried about its corporate interests and citizens in the UK, and for the 300,000-plus Brits in Spain. It “deeply regretted” Brexit, wanted a strong future relationship with the UK – but always saw EU solidarity, and the single market, as greater priorities, Llaudes said.

Luxembourg’s key concern was for an orderly Brexit, particularly in arrangements for the financial sector, said Högenauer – but like most EU27 members, the duchy was taken aback by the events that unfolded across the Channel.

If the early setting of the EU27’s red lines made it “very hard indeed for the UK to get a grip on any part of the process”, Zuleeg said, Britain really did not help itself: “Normally in negotiations you reach a compromise position, then present it to the other side. Britain couldn’t get to the compromise.”

So the government kept postponing any internal decision on its Brexit position. “It could not really use its civil service,” Zuleeg said. “It could not – a huge disadvantage in a negotiation – do any drafting of its own, only try to amend what the EU had already drafted. It could not, actually, negotiate.”

So the process so far has been determined by two key factors: the EU’s unity, and Britain’s fundamental, and insoluble, dilemma. “The EU forced it to choose between breaking the political promises made by Brexiters,” said Zuleeg, “or incurring significant economic harm. It couldn’t.”

It is a view echoed in several capitals. In Berlin, said Speck, there was “irritation and surprise at the chaos” across the Channel. “Everyone reads the UK papers here. We saw it all, every comment, every contradiction ... People were laughing – rolling their eyes – at some of the most extreme, the most arrogant, stuff.”

If the unpleasant rhetoric was amusing, the sheer length of time the UK took to publish its Brexit white paper was frustrating, and the ultimatums (“my plan or no plan”) and clumsy diplomatic attempts at divide-and-rule were annoying.

But it was the UK’s many and persistent delusions – on “frictionless trade” as a third country; on a special cake-and-eat-it deal; on avoiding a hard Irish border while leaving the single market and maintaining territorial integrity; on viewing the EU as a political arrangement, not a legal order – that have ultimately proved fatal.

With the deal sealed on the EU side, Zuleeg said he now expects it to pass through the House of Commons, possibly on a second vote. “The economic pressure will be enormous,” he said. “But we now have to negotiate the worst trade deal in history – the only one that’s a reduction on what we have.

“All the big issues: fisheries, Gibraltar, will come back with a vengeance, with the backstop hanging over us. It’s not over yet.”