THERE’S a story about the Suez crisis of 1956. Prime Minister Anthony Eden sent an urgent message to his commander in the field in Egypt. “Can you take Cairo?” After a pause back came the general’s reply: “Yes. Then what?”

The chief defect in the whole Brexit project is that it focused entirely on winning the referendum vote, and no thought whatsoever was given to the “then what” issue. And a crucial part of that was no one worried (or really even thought at all) about the border in Ireland.

The only land frontier between the UK and the EU was surely always going to be an issue that would need ironing out; and the criticism you will hear most frequently from Irish commentators is that this English “arrogance” has thrown politics into turmoil, about the most profound issues.

There’s a consensus among Northern nationalists and many in Dublin too, that Brexit has made a united Ireland, and therefore the break-up of the UK, a real live issue; and indeed a looming probability.

The Good Friday Agreement did a couple of things which bordered on the miraculous. The Troubles was never about religion – it was about identity. Are you British, or are you Irish? (The religious element came because Ulster protestants identified as British; the Catholics as Irish. )

What the Agreement did was to make the issue irrelevant – you could be whatever you wanted. Many began to identify themselves as “Northern Irish”, a unique, and completely new tag.

What Brexit has done is to destroy that – the Agreement assumed, reasonably enough, that both Ireland and the UK would remain in the EU.

The other hidden miracle was that it made Northern Ireland part of the UK – in perpetuity. The united Ireland issue, the “national question”, was off the table.

Now it is front and centre.

It almost defies belief that the main Unionist party, the DUP, thought Brexit would strengthen the Union. It makes one wonder if they ever understood the Agreement. Twenty years of peace and consensus thrown away, on a whim.

It is also beginning to become received wisdom that Brexit will drive Scotland out of the Union. What’s both Scots and the Northern Irish are asking themselves is this – do we really want to go on having our future decided by a) the English electorate; and b) by the English toffs baying loudest for Brexit. The corollary is – would we prefer to be in the European Union, or the British one?

We voted Remain, we cry. Tough, say the Leavers – it was a UK vote. Which in direct consequence, and inevitably, puts the UK at serious risk.

Tony Blair saw that the bonds of Union were becoming strained in the 1990s, which is why he brought in devolution.

It would have been unfair, went the reasoning, for NI to have an Assembly as part of the peace process, and Scotland and Wales not. Devolution strengthened the Union. If you disagree, just imagine was the independence referendum result would have been if the Scottish Parliament had not been in existence.

So now we come to “then what”.

A vote in Scotland to leave the Union would undoubtedly have a knock-on effect for Ireland, and it’s border. If England ignored that land frontier, Scottish voters should keep it in mind.

They should also know for what they’re voting. Nobody told any of us in 2016 we’d be worse off out of Europe. What’s going to be the hard financial cost? That, of course, may not be the decisive issue. If, in 1921, you had told the people of what was to become the Irish Free State that they would be economically much worse off out of the UK, for at least 50 years, what do you think their answer would have been?

No one has thought about unity in Ireland in a real sense – what persuasion has been offered to the Unionist population? Will the southern Irish, for instance, be prepared to get rid of the Tricolour flag, and have a new one to which Unionists could give allegiance?

Are Scots ready to lose the pound sterling, and maybe adopt the euro or an entirely new Scottish currency? Ireland had its own pound – “punt” – after 1921. It was tied to sterling until 1979, and was replaced by the euro in 2002)

How would a frontier with England work? In sum, let’s do the opposite of Brexit. Let’s think about “what next” before we vote on either independence or unity.

Denis Murray was BBC Ireland Correspondent from 1988 to 2008.