Here are three pro-European truths: the DUP is right, the SNP are not our friends, and our priority in fighting Brexit should not be our membership of the European Union. Unlikely they may seem, but each should be a key argument during the next stage of this purgatorial debate.

Let’s start with the DUP, whose MPs have shown singular consistency throughout this whole affair. In supporting Brexit, the party cleaved to the decades-old doctrine of its founding father, Ian Paisley, who opposed the European Economic Community, as it was called when he was elected to the European Parliament in 1979, and fought it from within.

But the DUP did not support Brexit in order to see its core belief, in the union of Northern Ireland with Great Britain, put at risk. Ever watchful, it could see what lazy Westminster polls could not: that by making a new border between the province and Great Britain, the withdrawal agreement threatens the logic and integrity of the Union.

The DUP is right. Ulstermen do not like the taste of fudge, and this confection was — for all the talk of temporary and unlikely measures — an attempt to sweeten a sour truth: that for the first time in centuries the UK would have an internal border. Not only would that destroy the integrity of our own internal market but it would threaten its logic too, so much so that a border poll would become inevitable, and such a poll might see Northern Irish people opt to remain in a union that they could depend upon rather than one they could no longer trust.

What is far less discussed is the next border that would be demanded — that favoured by the separatists of the SNP. For why, they would argue, should there be different customs and single market arrangements for Northern Ireland but not for Scotland, which voted to remain within the EU. If you can draw a boundary down the Irish Sea, why not between the Esk and the Tweed?

The purpose of the nationalists’ border would not just be to break further the UK’s internal market but — to paraphrase Robert Frost’s poem Mending Wall — to give offence. For the populists of the SNP dispute is valued above any reasonable compromise with the straw enemies of Westminster, and what better way to start an argument than a new economic wall?

If the SNP thinks the economic shock of a hard Brexit would best suit its ends, the Government’s deal is the next best thing. Now you may understand why the nationalists are so distinctly lukewarm about the emerging consensus that we may, as a United Kingdom, stay in a customs union with the EU.

All of this, you may think, would be central to the rationale for why Conservative diehards hate Mrs May’s deal. But in the topsyturvy world of Brexit, that supposition would be only partly right.

There are those who share the DUP’s concerns, which is why a switch by the DUP would have brought over many sympathetic Tory Eurosceptics. But let me reveal a dirty secret of the Conservative and Unionist Party: there are a number of MPs on the Tory benches who no more value the union of our four nations than they appreciate our membership of the European Union of 28.

If the cost of extraction from the EU is the end of our United Kingdom, that is a price they are willing — happy even — to pay to attain their ideological dream. It is why these same MPs would embrace a hard Brexit, even though it would put intolerable strain on the political and economic ties that bind our kingdom.

"They no more value the union of our four nations than they appreciate our membership of the EU"

All of this may seem strange to those of us who thought the Brexiteers wished to recover a lost age of British greatness. But then the most militant Brexiteers are not patriots but ideologues. What their patriotic fellow-travellers representing English seats fail to see is that were Brexit to force the union to lose one, and then two of its constituent parts, we would indeed return to a former time but one where England and Wales stood alone, a second-division power on the edge of Europe, incapable of generating the prosperity, providing the security and wielding the influence that England and Wales have as part of Great Britain.

That would be a loss of historic proportions. This is the greatest union of nations the world has seen, and the longest lasting. Our ability to provide for our own, to defend what is good and defeat that which is bad, to make a unique contribution to global culture and to play our part in the story of humanity is incalculably greater than it would have been if we had plodded along on our own for the past 300 years. Were Brexit to precipitate the break-up of our own union, then we would have turned a disaster into a tragedy.

Before we make that mistake, it would be as well to remember what makes the United Kingdom so special and the vandalism of populist nationalists so repellent: that the essence of our union is that we achieve more together than apart, and that in unity we are better able to celebrate difference.

When demagogues around the globe seek to create dispute, our union speaks of something beautiful and, in the current climate, rather radical. It is why the United Kingdom still has something important to teach the world. In deciding not to heed our own lesson, we may, via Brexit, undo Great Britain.