That's why it's so hard to talk about an agreement with Northern Ireland after Brexit

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There was immigration. There was excessive regulation. There was the challenge of the principle of parliamentary sovereignty. But at no point in the EU's referendum campaign has there been an idea that Northern Ireland would become the issue on which Britain's withdrawal from the European Union would eventually turn.

Yet we are there. At five months from the departure of the United Kingdom, a withdrawal agreement is in force, with the exception of the question of what happens to the Irish border. The two sides can not agree on the so-called Irish support – the system that would come into effect if their trade agreement after Brexit did not produce a solution that would prevent a hard border.

The question, simply, is this. The UK and the EU are committed to avoiding the need for a border between Northern Ireland and the Republic. Indeed, the United Kingdom has gone further, underlining its commitment to "avoid a rigid border, including any physical infrastructure or related controls and verifications".

Until here everything is clear. However, while expressing its opposition to borders and infrastructure, the British government clearly expressed its determination to extract the United Kingdom from the single market and the customs union. The problem is that it involves the need for two types of controls and controls: customs (incorporating a host of things, including origin rule checks) and regulators (to ensure that each side meet the necessary standards).

The question then becomes: where should these controls be carried out? And here we hit (again) another roadblock. The background of the European Union would put the new frontier at sea in Ireland. Northern Ireland would remain in the customs union and the single market for goods, which would involve customs and regulatory controls in the United Kingdom.

And this, it seems, is unacceptable not only for the Democratic Unionist party, but also for a number of other deputies, including conservatives far removed from the European research group.

So how do you make this particularly circular circle coincide? The EU knows what it wants and will not change. He wants the support of Northern Ireland to be included in the Brexit withdrawal agreement, which must be settled by March 29, 2019. He will not agree to fix a time limit for the operation of this agreement (if there was one if safety net).

What the EU, up to now, has not wanted to accept, is the idea that support is applied to the whole Kingdom. -United. This, he says, is something for negotiations on future relationships, so not something that can be included in the withdrawal agreement.

Given what the EU wants, the UK government can sell a security agreement only if the future relationship agreement removes the need for that support. It is only through this relationship that Westminster can obtain the assurance it needs that EU support insists that its membership will never be used.

So we go back to the old sequencing problem. In the short term, the withdrawal agreement will be finalized before a political declaration on future relations. But, politically, it is crucial for London that both appear in public at the same time because it will make the first one more acceptable. The political declaration will also be just that – a statement. It will not be a legally binding text. The future relationship itself will only be formally negotiated and agreed upon when the UK leaves the EU.

So the question is to what extent both parties come to the conclusion that by signing a waiver agreement containing the backstop, the UK is actually signing a blank check. This is what they will probably try to use positive language ("we are convinced that we can find a way to forge a future relationship, which means that support will not be needed").

But the type of solution envisaged by London will be difficult to find. In principle, technical support from UK Customs would be acceptable to the EU – but not if the UK insists from the outset to set a deadline. Why should the EU negotiate something that the UK does not really want?

And then there is the thorny problem of the rules. The Checkers propose to solve the Irish problem by proposing a customs plan (which is not acceptable to the EU, but leave it aside for the moment) and what the government has described as a "common rule" under property and agriculture would remain aligned.

The problem is that the EU does not really like the idea. And without that, a large number of MPs (and not only those of the DUP) have a hard time believing that part of the UK will be subject to laws on which Parliament does not have a voice. .

So, again, the language will matter. London still thinks he can play chicken with his partners. The announcement of what is called full-fledged parliamentary preparations for a Brexit without an agreement starting in the second week of November was aimed, at least in part, at confronting the Irish with a dilemma. He does not want to insist on a backstop to end up without any agreement. The hope in Whitehall is that Dublin can put pressure on its EU partners to give more in the political declaration than they would otherwise have.

Even if they do, of course, the fact remains that the declaration will not be binding. MEPs committed to an agreement guaranteeing that the security system will never be used risk being disappointed. That's when – when the debates of Parliament and the vote on all that will be agreed – we will know if Ireland turned out to be the Brexit circle that really could not be squared.

This article was first published by the UK in A Changing Europe.