If this feels like a dreadfully fractious period in EU-UK relations - vaccines, the Northern Ireland Protocol - don't forget the rollercoaster of autumn 2019.

In his first Commons announcement as prime minister, Boris Johnson had called for the "abolition" of the Irish backstop, then still the only mechanism on the table to avoid a hard Irish border.

In the downwash of the Johnson helicopter, as it lumbered its descent onto the Brexit scene, there was rancour in parliament, eye-balling with the EU, threats of no deal.

The Labour Party was preventing Johnson from holding a snap election, Dominic Cummings moved into Downing Street, and the Supreme Court declared as "unlawful" Johnson's shutting down of Parliament, in order to keep the threat of no deal on the table.

Johnson was insisting there would have to be a customs border on the island of Ireland, albeit streamlined by "alternative arrangements".

On 1 October, RTÉ News reported that the UK was proposing a string of "customs clearance sites" along, but 5km-10km back from, the Irish border. A week later Angela Merkel told Boris Johnson by phone that the EU would not accept such a border on the island of Ireland. No deal was starting to look like a runaway certainty.

However, the ice was beginning to crack. Four days previously, Johnson said the UK could accept an all-island arrangement for goods, including agri-food products.

"[We] propose the potential creation of a regulatory zone on the island of Ireland covering all goods, including agri-food," he told the House of Commons. "For as long as it exists, the zone would eliminate all regulatory checks for trade in goods between Ireland and Northern Ireland."

This was remarkable.

Although he still proposed a customs border, the implication of Johnson's offer was a regulatory border on the Irish Sea.

Yet on 19 August, Johnson had warned European Council President Donald Tusk in a letter that such a scenario would see Northern Ireland "gradually detached from the UK economy across a very broad range of areas," an outcome that was "unacceptable to the British government".

What broke the deadlock was the Thornton Manor meeting on 10 October.

The encounter between Johnson and Leo Varadkar was a straightforward barter: Johnson would accept a border on the Irish Sea for SPS and customs; Varadkar accepted that the Northern Ireland Assembly would have to give its consent every four years.

The Northern Ireland Protocol was revised, the Withdrawal Agreement clinched, no deal was avoided, and Johnson won an 80-seat majority for his troubles.

We are still living with the consequences of that deal. When the EU this week launched legal action against the UK for the second time, officials in Brussels went straight back to that moment in the autumn of 2019.

"The UK government and the then chief negotiator [David Frost], who is now Minister of State, made a choice in September 2019," said an official. "Then there was a renegotiation of the Withdrawal Agreement based on the premise that there would be checks on some goods moving east-west, from Britain to Northern Ireland.

The official added: "That was really crystal clear to all sides, and it was the working assumption of the talks at the time. We're… dealing with the consequences of the choice of Brexit and also the content of the protocol."

In other words, the UK has always known there would be controls on goods crossing the Irish Sea, and by insisting on a hard Brexit the UK chose to make those controls more onerous than they might have been.

So where do things stand?

On 17 December, the EU and UK agreed a series of flexibilities on the protocol, including a three-month grace period delaying the need for cumbersome and expensive paperwork for supermarket consignments, a six month suspension of the ban on chilled meats, including sausages, and a three-month grace period governing the movement of express parcels.

On 3 March, the UK announced it was unilaterally extending the three-month grace period for supermarket paperwork from 1 April until 1 October. It would also lift the ban on certain plants and plant products with soil attached, and the restriction on the movement of agricultural and forestry machinery, unilaterally waiving the need for EU phytosanitary certificates.

London also extended the grace period for parcels by a further six months.

So, we are now in a situation where the UK has told Northern Ireland supermarkets, food suppliers, farmers, nursery centres, and consumers to keep doing what they were doing, and that things will remain the same (until "at least" 1 October), while the EU - with whom the UK concluded the international treaty containing the protocol - has said that such a scenario is illegal.

Come 1 April, Northern Ireland civil servants - and supermarkets - will be in a legal quandary. Are they breaking international law? Or do they only have to comply with UK law?

Under UK law, the effect of an international treaty has to be transposed into national law. That is why the EU insisted that the Withdrawal Agreement contain a commitment on the part of the UK to take the internal legal measures necessary to give effect to the provisions of the Northern Ireland Protocol. 

Northern Ireland civil servants were already in a legal quagmire when Minister for Agriculture Gordon Lyons ordered work to be stopped on permanent Border Control Posts - mandated by the protocol - at Larne and Belfast ports.

Legal advice has been sought, and discussion papers are being prepared for the Northern Ireland Executive, but the issue is still not settled. Officials can take legal advice, but ministers take decisions. If it's a DUP minister, he or she will argue that if the protocol is going to be removed then the buildings are not necessary.

Yet, there is no indication that the protocol will be scrapped.

With its launch of an infringement procedure against the UK, and a second track potentially leading to arbitration through the Withdrawal Agreement, the European  Commission believes EU law applies to both supermarkets and civil servants in Northern Ireland.

In his letter to his opposite number David Frost, the European Commission vice-president Maroš Šefčovič said the UK authorities "had authorised individuals to disregard [European] Union law even though it is directly applicable to them".

All of this is unfolding in the midst of a poisonous political climate in Northern Ireland, in which loyalists have withdrawn their support for the Good Friday Agreement, and a slump in trust between the EU and UK.

Is all hope lost? 

Not entirely. The latest UK move - to many in Brussels redolent of the Internal Market Bill last September - has certainly angered the EU, and put the Irish Government, which had been attempting to gently coax member states and the commission into taking a flexible approach to the protocol, in a difficult position.

"We were, fairly unapologetically, trying to act as an interpreter between the British and EU side," says a senior Irish Government source. "Of course, the legs have been whipped out from under us by the British manoeuvre because it was done unilaterally. Obviously, we have no choice now but to double down on the EU side."

On the EU side, member states are balancing the need for flexibility with a growing conviction that the UK will periodically go "rogue" in order to secure more concessions.

That has hardened attitudes in capitals. When the issue was discussed by EU ambassadors last week, a number of diplomats wondered if Brussels should start thinking about the leverage it has, including the granting of equivalence for financial services.

"They were not threatening to withhold an equivalence decision," says one diplomat present. "Nevertheless, the suggestion was made that it is hard to do business if [the UK] basically refuses to implement [the protocol], and, as with the Internal Market Bill, tries to extract things by going on a rogue path.

"Instead of offering the other cheek, the commission should think again about what kinds of elements we have, if we are really living in a world of distrust and non-compliance," the diplomat said.

UK sources have been relatively sanguine about the legal action.

The claim is that back in December it was acknowledged by both sides that the three-month grace period for supermarkets to adapt to the new regime of export health certificates for agri-food products would not be long enough.

"The reality is that [compliance] has proved more difficult to achieve than when it was signed up to," says a UK source. "The other reality is that when it was signed up to, people recognised that we would probably need more time.

"I don't think either the EU or UK were completely confident that the April 1 deadline would work and there was a recognition we might need to return to this. The triggering of Article 16 also intensified feelings towards the protocol in Northern Ireland, so it was pretty essential that we took action to stabilise the situation."

That is not the view in Brussels, and it was certainly not what cabinet office minister Michael Gove told the House of Commons on 9 December. Then the UK co-chair of the Joint Committee said that three months should be enough for the "one or two" supermarkets who weren't ready to get ready.

However, now London insists that since supermarkets need a few weeks to order food consignments in advance, and since the EU still had not agreed to extend the grace periods, supermarket shelves would have started to empty from 1 April.

"The fundamental point is that if the grace period had not been extended then quite large volumes of product would no longer be able to go from GB to NI," says the source, "so there would be an impact on the food supply in Northern Ireland. The UK government can't be in a position where supermarkets in NI have shortages. It would be noticeable and would really not help with the situation on the ground."

This is part of a familiar pattern. The UK points to what is happening on the ground, says it cannot allow supermarket shelves to go bare, and alludes to the commission's move to invoke Article 16 in January; the EU points to the reality of what the UK agreed to 17 months ago, and says, you knew all this and yet you didn't prepare.

On Article 16, one EU official says: "The EU did not invoke Article 16 of the Protocol. No [European] Union institution has proposed or adopted any measure that would require taking recourse to that provision. The mere consideration of including a reference to that provision was a mistake, and senior politicians on the EU side, including the [Commission] President [Ursula von der Leyen] herself, have expressed regret and apologised for this in public."

That being said, there is a narrow political space for things to be salvaged. UK officials say that the unilateral action was not unlawful because London was not abjuring the protocol. Rather, it was taking practical steps to ensure that acute hardships were avoided, but while on the pathway to full compliance.

Furthermore, both sides have highlighted the importance of getting back into the processes provided for by the Withdrawal Agreement - the Specialised and Joint Committees - in order to fix things.

Indeed, there were contacts between EU and UK officials in the second part of this week.

The focus has been on what Maroš Šefčovič referred to in his letter to David Frost last Monday as a UK "roadmap" that would set out clearly, with milestones and "deliverables", as to how the UK intends to comply with the protocol, and with December's agreement.

Can a solution to the standoff be found?

"Ultimately, a solution would have to be based on a comprehensive set of understandings of all the issues that remain to be resolved," says one EU official, "a comprehensive understanding of the steps that would lead to their resolution by the UK in a satisfactory manner. That's what we are referring to when speaking about a roadmap."

One EU diplomat is cautiously optimistic, but is clear that until the roadmap is delivered, then a Specialised Committee followed by a Joint Committee meeting - where the high-level political decisions are taken - will be on hold.

"It's not clear yet what or when or how this roadmap will appear," says the diplomat, "but a formal meeting of the Joint Committee or Specialised Committee is contingent on how this roadmap develops.

"But the mood music has improved. It's civilised, and both sides are engaging."

Part of this choreography could involve a meeting between Šefčovič and Frost, but not at Joint Committee level. "That's all contingent on the level of commitment of the UK to come into line and how it demonstrates that," says the diplomat.

While the EU wants a credible roadmap, the UK is pushing its own "operational plan" to solve the protocol's main sources of tension. This would involve the UK providing financial support to supermarkets to develop a digital traceability scheme that would reduce the burden of compliance with EU food safety rules.

Once the grace period ends, suppliers to Northern Ireland supermarkets would have been required to provide perhaps hundreds of export health certificates for mixed loads of agri-food products, with each cert signed off by a vet.

The UK's operational plan is designed to streamline and make cheaper that process, so that instead of an individual certificate signed by a vet for every product, one vet could digitally sign off the entire load.

UK sources say they are willing to discuss the details, and the commission has signalled that it wants workable solutions. But the roadmap comes first.

"Our assessment of the [UK] operational plan, which we've received, so far is that it will not lead to full compliance with the protocol and, as such, it is not sufficient," says an EU official.

Another senior diplomat says: "With or without legal action, the idea was always to come to a solution that would probably have consisted of some time limited extensions to the grace periods, plus credible commitments from the UK to really put in place what they've signed up to."

It is highly unlikely, however, that the choreography will be complete by next Friday, so there will be a period of legal limbo from 1 April.

Since the grace period was underpinned by a promise by the UK to remain aligned with EU SPS rules, at the very least the UK will have to keep that alignment going, even if Brussels regards London's extension as illegitimate.

But it's clear the UK will not reverse its extension.

"The reason the announcement was made was because supermarkets require certainty in order to continue to make deliveries to Northern Ireland," says a UK source, "and that position must stay. And therefore the grace period had to be extended."

Despite these tensions, there is a clear but narrow path to getting things back on track. Ironically, the UK's go-it-alone approach has simply made the path more treacherous, due to the lack of trust on the EU side.

One keen observer, close to developments, notes that in October 2019, Boris Johnson was desperate to secure a deal on the Withdrawal Agreement so he dramatically reversed course on Northern Ireland, and established a border on the Irish Sea that he said a British government could never accept.

In December 2020, Boris Johnson was, again, desperate to secure a free trade agreement, but the Internal Market Bill (IMB) was an obstacle.

Johnson knew he could not remove the IMB clauses unless Gove and Šefčovič reached a deal making the protocol more flexible through grace periods, etc.

"They went for a deal at any price and they were incredibly relieved to get what they got," says the source.

Critics say that for the second time, Johnson has entered a grave undertaking, either not fully understanding it, or believing London could renegotiate later.