As the row over EU nationals dominates the opening of the Brexit negotiations, Theresa May is once again coming under attack for her refusal to offer them a unilateral guarantee that they be “allowed to stay”.

George Osborne has just alleged, via his newspaper, that the then-Home Secretary single-handedly stopped David Cameron from doing so in the immediate aftermath of the Brexit vote.

With the EU’s negotiators taking a dim view of what the Government considers to be a “big and generous” offer, the idea seems to be developing that the Prime Minister could have avoided the whole row by taking the high and generous path in the first place.

Yet this is not the case. In fact, by exposing the depth and complexity of the issues at involved the current dispute actually suggests three important countervailing points: that a broad-strokes guarantee would not have avoided the present showdown; that the EU made working out a detailed guarantee before now basically impossible; and that such concessions would have been an extremely ineffective, even counter-productive, negotiating tactic.

The Government’s primary reason for not making the big headline concession is that they expected full reciprocity from Brussels and had not received it to their satisfaction. One may contest the details of that claim but it seems bizarre to dispute the principle, unless the Government is expected to defend the rights of British nationals in Europe less vigorously than those of EU nationals in the UK.

But the problem is actually broader than that. Offering EU nationals genuine peace of mind about their future in the UK means not just promising that they won’t have to leave – although that is important – but spelling out the details of the terms of their future residency.

The Prime Minister’s outline offer goes some way towards doing this, intending as it does to treat EU nationals as Britons for the purposes of work, healthcare and welfare. Yet many other important details are currently the subject of fierce debate.

For example, the UK wants people to have been resident for five years (prior to Brexit in 2019) to qualify for this protected status, whilst the EU wants people who moved to Britain ‘in good faith’ in May 2016 to qualify.

Then there’s the question of spouses. Currently EU nationals actually have an easier time bringing them into the country than British citizens, as the latter have to meet an income threshold. Naturally Brussels wants this artefact of freedom of movement to continue post-Brexit, but London is deeply resistant for fear of turning EU nationals into a privileged group within the UK.

And whilst the Government proposes to treat EU nationals as Britons for welfare purposes, that still opens up disputes – for example, over whether they will still be able to send child benefit to children outside the UK. Brussels says yes, London no.

This is before we even get to the constitutional dispute at the centre of the current row: the future role of the European Court of Justice.

Politico reports that the reason Britain and Europe are “on a collision course” over the rights of EU citizens, despite the UK offering to treat them as British for the purposes of work, healthcare and welfare, is that Brussels’ insists on the ongoing rights of EU nationals being guaranteed by the ECJ, rather than the British justice system.

Preventing this is a red line for London, which insists that any disputes arising over EU nationals in Britain post-Brexit should be worked out bilaterally with Brussels. This is eminently sensible, and not just because ending the jurisdiction of the ECJ over British courts is a foundation stone of Brexit.

Maintaining the jurisdiction of an EU institution over a state outside the EU resembles the liberal order the Union is supposed to embody much less than the straightforwardly imperial style of ages past. Don’t take our word for it, either. In this eye-opening piece Franklin Dehousse, a Belgian former ECJ judge, sets it out:

“In principle, it is hard to justify that EU migrants, through the maintaining of many European regulations, will become some sort of a super-privileged caste in the future UK (as will the UK migrants in the EU Member States). The country will thus become some kind of new 1930 Shanghai, where the EU citizens will benefit from multiple privileges (the more so if the European Court of Justice keeps a full jurisdiction).”

Beyond the narrow issue of EU nationals, the very principle of the future role of the ECJ in UK-EU relations proposed by Brussels is even more problematic:

“The UK would then become the only third state submitted to the full and direct jurisdiction of the ECJ. Furthermore, one wonders how this is considered acceptable for a sovereign state. Such a state would thus be bound by decisions taken by a judicial authority where it is not represented and whose judges would be appointed by its potential opponents! This time, the solution looks a lot like the leonine treaties imposed by England to China in the 19th Century.”

Put all this vital context in place and its clear that a broad-strokes concession about EU nationals’ rights would not have avoided this fight. The disputes above are all on important matters of implementation detail and would have arisen even if the UK had guaranteed ongoing residency in principle.

It’s also obvious that a detailed guarantee was impossible to deliver without talking to Brussels unless we wanted to simply announce fixed positions on all the above disputes, which would have been a very aggressive way to proceed.

As the EU refused to talk to the Government, even unofficially, until the formal negotiations started on Monday – Angela Merkel rebuffed May’s offer to find a solution in November last year –  an early deal on the details was thus impossible.

Finally, the aggressive strategy we see Brussels’ pursuing – De Housse calls their demands “manifestly excessive” – in the above disputes gives us no reason to think they’d have done anything with such a concession from London but bank it – and then use the fate of British nationals in Europe as leverage on all these issues and more besides.