The border has been invisible since the signing of the Good Friday agreement in 1998 ended 30 years of bloody conflict between the Protestant and Catholic communities in Northern Ireland.
Three thousand people had been killed, but the situation had reached stalemate. The Good Friday deal let both sides accept that fact.
For the (Catholic) nationalists in Northern Ireland, a completely open border with the (Catholic) Republic was a vital part of the deal. It implicitly acknowledged that the two parts of the island might one day be reunited ... although not just yet.
As the 1998 agreement plainly said, people born in Northern Ireland have the right to be "Irish or British or both as they may so choose".
And it worked ... sort of. The only way you can tell you have crossed the border now is that the speed signs change from miles to kilometres or vice versa.
It was a brave, imaginative deal that has given Northern Ireland 20 years of peace, but it is now at risk.
When the "Leave" side narrowly won the Brexit referendum and Theresa May replaced David Cameron as British prime minister in 2016, she had a credibility problem.
Like Cameron, she had supported "Remain", but the Conservative Party she now led was dominated by triumphant Brexiters.
So she became an enthusiastic Brexiter herself.
The English nationalists who ran the Brexit campaign had said nothing about leaving the EU's single market and customs union, but within weeks of taking office May declared that Britain must leave both of them.
She even made this demand part of her famous "red lines" non-negotiable minimum that the British government would accept in the divorce settlement.
Unfortunately, ending the customs union would mean re-creating a "hard" border between Northern Ireland and the Republic – and that might lead to a renewal of the sectarian civil war between Catholics and Protestants in the north.
It's not clear when the Conservative government in London realised that the Irish border was going to be the biggest stumbling block on the road to Brexit, and the party's more extreme Brexiters are still in denial about it.
But the Republic will stay in the EU, and it insists that there must be no hard border after Brexit. Ireland has seen enough killing.
No hard border is, therefore, an EU red line, and it's impossible to square that with May's decision to leave the EU customs union.
If there is no customs union, then there have to be border checks — and maybe a new war in the north.
So the EU suggested a backstop. If London and Brussels can't come up with a free trade deal to keep the border soft (i.e. invisible), then Northern Ireland could stay in the customs union, and the rest of the UK could leave.
The real border, for customs purposes, could run down the middle of the Irish Sea.
Theresa May actually signed up to this solution last December because the only real alternative was a hostile Brexit that simply ignores the EU's position.
But no sooner had she agreed the backstop with the EU than rebels in her own camp – extreme Brexiters and members of a small Northern Ireland-based Protestant party whose votes are all that keeps the Conservatives in power – forced her to repudiate it.
Now May's position is pure fantasy — no customs border with the EU either on land or in the Irish Sea. Which is why the probability of a chaotic "no deal" Brexit is growing daily, and the prospect of renewed war in the North is creeping closer.
Is renewed war really possible?
Last year Sinn Fein, the leading Catholic party in Northern Ireland, withdrew from the power-sharing government mandated by the Good Friday agreement.
That could be seen as clearing the decks for action once it became clear that Brexit would
undermine all existing arrangements in Ireland.
And if the UK crashes out of the EU without a deal, the ratings agency Standard and Poor's predicted on Tuesday, unemployment in Britain would almost double, house prices
would fall by 10 per cent in two years, and the British pound would fall even further.
First impoverishment for the British, then war for the Irish.
*Gwynne Dyer's new book is 'Growing Pains: The Future of Democracy (and Work)'
