The threat of a 'hard' Irish land border is the only substantial bargaining ploy available to Britain's Brexiteers. As in 1912-22, Ireland is again pivotal to a crisis of the British state, but now unionists - not nationalists - are key players at Westminster, and it's a crisis too for the Irish Republic and the whole EU.
Despite knee-jerk responses from Sinn Fein demanding a border poll and from Democratic Unionists claiming that an 'Irish Sea border' undermines political union with Britain, Northern Ireland's constitutional position is not the issue.
The necessary poll is very unlikely, and majorities North and South for a 'united Ireland' are even less likely given Brexit uncertainties - as SF now seems to realise.
Yet there appears to be no recognition - especially not from the DUP propping up the Brexit Government - that Northern Ireland, along with the Republic, is being 'held to ransom' in the probably doomed attempt by Brexiteers 'to have their cake and eat it'. That, and the damage here if they don't get their way, is the real and immediate issue.
Ireland's borders are central to the resolution or the deepening of the crisis - unless the UK has customs union access to the single market which would get rid of the border problem at a stroke.
It is possible to distinguish four other scenarios. The first two were outlined in the Belfast Telegraph (August 4), and the second two arise from the UK's recently published negotiating stance on the border. It has been variously seen as incoherent, stupid, clever or too clever by half, but cannot be read simply at face value.
SCENARIO ONE: The disaster of a 'hard' Irish land border by default.
It will happen unless a proper alternative is agreed. Briefly, it's a disaster waiting to happen: economically, politically and socially destructive, attracting paramilitary attacks on border posts and creating a smuggling bonanza.
For it's an inherently leaky border, and the de facto secure borders would be the seas around Ireland, with checks at ports and airports in Britain and the continental EU (eg to exclude non-Irish immigrants, and sub-standard foreign goods, respectively).
SCENARIO TWO: Northern Ireland in a customs union with the EU: the sea borders solution
Again briefly, instead of a single 'hard' border, the EU and the UK can be separated by two 'soft' (but actually more secure) ones with Ireland in between them.
Its position would be like the space between the double security doors for entering and exiting banks, except here one 'door' links Ireland with Britain, the other with the continent.
This safeguards the existing movements between Ireland and both Britain and the continent, except for people and goods which originate outside Ireland.
Northern Ireland needs single market access.
As once acknowledged by Arlene Foster (though before the RHI scandal dented her power within the DUP), "Northern Ireland could have a different relationship to the EU's single market, or customs union, from the rest of the UK following its exit from the EU" (Reuters October 29, 2016).
That inevitably requires sea borders of some sort. It's also the Irish Government's default preference if the UK as a whole does not have access to the single market.
SCENARIO THREE: The UK's new customs union relationship: no border problem
Rather than actually being in a customs union, Britain's Brexit negotiators want to 'mirror' EU procedures, and therefore also be able to do their own trade deals where goods would not meet EU standards, but they promise not to export these on to the EU. However, this would be a very complicated, unprecedented arrangement, requiring a lot of trust and goodwill - currently in short supply. It seems destined to fail.
But Brexit negotiators hold a substantial bargaining chip, and, unfortunately, we are it. Their scheme removes the border problem with the non-too-subtle threat that the alternative is the (British-created) Scenario One disaster.
If they don't get their way, we and the EU suffer the damage. Cynically, they have waxed lyrical about not wanting a 'hard' border, knowing full well that they are creating circumstances which could force the EU to insist on a 'hard' land border to protect the single market.
Knowing too that immigration is better controlled at Britain's ports and airports, the UK negotiators have shown no interest in creating their own checks at the Irish land border.
Much of the criticism of their stance is misplaced. If it's 'incoherent' that is because of their ulterior motives: holding a friendly neighbouring state and its own Northern Ireland citizens to ransom might seem hostile or lacking in goodwill, and is better hidden by 'spin' and obfuscation.
Far from being 'stupid', their ploy is actually cunning. But the DUP leaders (Nigel Dodds and Sir Jeffrey Donaldson more than Mrs Foster, who comes from a border area) have been too easily persuaded by the nonsense of a 'frictionless' land border. DUPes selling their Northern Ireland 'birthright for a mess of pottage', or unaware they are collaborating in it being held hostage. They missed the bigger picture, whether because of simplistic notions of sovereignty (shared with Brexiteers in Britain), or exaggerated worries of becoming 'semi-detached' from Britain by a sea border (NI is already 'semi-detached' without the union being endangered, and they don't seem to worry about 'semi-detachment' when supporting different NI legislation on 'moral' issues).
SCENARIO FOUR: A 'hard' border through crashing out of the EU by design.
A 'hard' border could occur by default - British negotiators 'too-clever' and unrealistic. But there's the 'nuclear option' where more 'realistic' (or extreme) Brexiteers calculate that Scenario Three has no chance and plan to 'crash out' by collapsing negotiations with the EU and holding it responsible (what will they do without it?).
This offers them the possibility of a 'hard', decisive Brexit which they don't have a majority to achieve democratically. For some, it cannot happen soon enough. They fear compromises in a long 'transition' and want an early exit which stops them, restoring their independence to again stride the world making deals.
But this is where Brexit really does unravel. Making their own deals while retaining access to the single market has the logic of additional trade possibilities (if the EU would allow it).
However, giving up their privileged access to the world's largest trading bloc, and imagining they can then make better deals than it can, make up the difference in lost EU trade and make additional gains, is seen by most observers as delusional, the realm of imperial nostalgia and neo-liberal fantasy.
The deals could be 'better' only in the sense of including goods not up to EU health and safety standards; an 'independent Britain' desperate for deals would be vulnerable to them.
It might, for example, accept lower standards in food imports from the Americas which would provide cheaper food. But that could damage British and Irish consumers and farmers.
It could scupper Britain's Scenario 3 plan of 'mirroring' the EU customs union, given the scepticism about separating out goods which don't comply with EU standards.
The UK as a whole being in some customs union arrangement would remove the border problem, but the 'hard' land border now seems more likely.
Virtually everyone on the island (smugglers and paramilitaries excepted) would suffer - North and South, unionist, nationalist and neither. Scenario Two, the sea borders solution, offers a clear alternative and the damage-limitation or disaster-prevention campaign should start now.
James Anderson is Emeritus Professor of Political Geography in the Mitchell Institute and a founder-member of the Centre for International Borders Research at Queen's University, Belfast