A border is coming to the British Isles – just where, though, is not yet certain. This weekend, the leader of the Irish Republic’s nationalist Sinn Fein party called for Northern Ireland to begin preparing for a border poll and a united Ireland. Sinn Fein has been making such calls ever since the Brexit referendum, but only recently have they gained some credibility. The United Kingdom’s imminent departure from the European Union is poised to disrupt the trade and regulatory harmonization that has made the illusion of a united Ireland – and thus the Good Friday Agreement – possible. If the U.K. leaves the EU’s customs union and single market as promised, or leaves the EU without a withdrawal agreement, a border somewhere between the U.K. and the EU is inevitable. The decision of where to place that border will reshape the British Isles and their relationship to the Continent.

The Backstop

The Irish Republic and the EU have been adamant that there cannot be a border in Ireland, arguing that it would undermine the Good Friday Agreement. (That agreement brought a near-total cessation of the civil war between republicans, who seek a united Ireland, and unionists, who wish to remain part of the United Kingdom.) So they proposed the so-called Irish backstop. The backstop would effectively keep Northern Ireland in the EU’s single market, while both Northern Ireland and Britain would remain in a customs union with the EU “unless and until” they reach an agreement that removes the need for the backstop. (The EU’s preference was for the backstop to apply only to Northern Ireland, but the British government insisted on this U.K.-wide format.) Northern Ireland’s Democratic Unionist Party has fiercely opposed the backstop and any other solution that would treat Northern Ireland differently from Britain. The DUP’s objection would matter less were it not for the fact that British Prime Minister Theresa May’s government depends on the party’s support for a functional majority.

Unionists and others in Britain see the backstop as nothing short of an Irish and EU plot to seize a piece of the United Kingdom as the price of Brexit. Northern Ireland wouldn’t be under the control of the Irish Republic, but it would be forced to apply the EU’s customs and regulations without a voice in shaping them. Ireland and the EU argue, however, that the backstop is about preserving the status quo and avoiding the need for a border between the two territories that could incite violence.

The EU is not especially thrilled to be in this position – the backstop is the biggest obstacle to securing an orderly Brexit, though not the only one – but it has been loath to pressure Dublin to abandon its stance. For Brussels, the only outcome worse than a no-deal Brexit would be abandoning a small member state on an issue that state views as existential just to reach a deal with London. Brussels fears this would send a signal to other small EU member and prospective states with potential territorial disputes (particularly Baltic and Balkan countries and Cyprus) that Brussels cannot be relied upon to defend them.

As for Ireland, on the surface, its dogmatic insistence on the backstop looks like a contradiction: If what Dublin fears most is a border, and failure to reach a deal ends in a border, why not yield? Agreeing to a five- or 10-year limit would at least kick the can down the road. But there is a difference between a border that arises as a result of failed talks – talks that Ireland and the EU are confident would have to be picked up sooner than later – and a border that arises with Ireland’s acquiescence. If Dublin admitted in principle that it could accept a border with Northern Ireland, it fears that such a border would emerge and would be permanent, crushing the centuries-old dream of a united Ireland. A border resulting from a disorderly Brexit, on the other hand, is seen as temporary; implementing the backstop would likely be the EU’s first condition to start talks on an EU-U.K. trade agreement.

It’s a position that works in principle but is explosive in practice. In a January survey by the Irish group Red C Research, 70 percent of respondents in the Irish Republic opposed a hard border. The government in Dublin has struggled mightily to explain what it would do in the event that there is no withdrawal agreement. After months of insisting there were “no plans” for a border, the Irish deputy prime minister said vaguely in mid-January that a border “could be at sea.” This presumably meant in the Irish Sea, not between the British Isles and the Continent. Irish Prime Minister Leo Varadkar referenced the latter idea during a private meeting with opposition party leaders, saying that all goods from the U.K. and Ireland could be checked at Continental ports – an outcome he said must be “avoided at all costs.” At the World Economic Forum’s meeting in Davos at the end of the month, however, Varadkar took his hardest line yet, saying a no-deal Brexit would result in “customs posts,” other physical infrastructure and “possibly a police presence or army presence to back it up.”

In trying not to inflame tensions at home, the Irish government is alienating the EU. To this point, Dublin has held outsize influence in Brexit negotiations because it had EU backing, but it risks being crushed between the U.K. and the EU if it refuses to control the EU’s external borders in a no-deal Brexit. The EU has two overarching objectives that would be put at risk in a disorderly Brexit: protecting the interests of a member state over a non-member state and protecting the single market. If the Irish Republic fails to defend the single market, the EU would be forced to choose between its objectives – and it would choose the single market. This would effectively mean that the Irish Republic would be expelled from the EU trade area and forced into a bloc with the United Kingdom. Ironically, escaping this bloc would be all but impossible without reaching a resolution with the U.K. or unilaterally installing border checks – it would essentially be Ireland’s Brexit. For a nation that has historically looked to Europe for allies in resisting English domination, being trapped in such a bloc with the U.K. would be a significant strategic failure.

The Poll

Ireland’s last option might just be a border poll. Under the terms of the Good Friday Agreement, the secretary of state of Northern Ireland is to call a referendum on union when it appears likely that a majority would support a united Ireland. (The agreement allows polls to be held every seven years until it passes.) The imprecision of this language was never a serious concern, because a majority always seemed a generation away. Belfast-based pollster LucidTalk said polls have historically shown support for remaining in the U.K. in the low-to-mid 50s when undecideds are removed.

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But the question has changed since Brexit, because remaining in the U.K. now entails leaving the EU. In December, LucidTalk asked Northern Irelanders how they would vote in a referendum on a united Ireland in the event of a no-deal Brexit. Fifty-five percent of respondents, including 11 percent of unionists, said they would certainly or probably vote to leave the United Kingdom. Removing undecided respondents pushes the figure up to 57 percent. Sure, this is just one poll, but 56 percent of Northern Ireland’s voters backed “remain” in the 2016 referendum. And the findings make some sense: The threat of a border spurs moderate republicans who have otherwise preferred to avoid the disruption of a border poll to come out strongly in favor of one, while neutral voters like those who support the Greens or the Alliance Party, who would likely be the decisive votes in a border poll, are fiercely in favor of a united Ireland over a no-deal Brexit, according to LucidTalk’s findings. (Both the Greens and Alliance lobbied for “remain” in the Brexit referendum.) It’s also notable that the 2017 Northern Ireland Assembly election, the only one since the Brexit vote, marked the first time in Northern Ireland’s almost 100 years of existence that unionists failed to reach a majority.

Our assessment from the jump has been that there will be a withdrawal agreement, and that is still the likeliest scenario. It’s in the interests of all sides, and failure would be devastating – and not just economically. But should a no-deal Brexit appear imminent, it is faintly conceivable that public outcry in Northern Ireland could trigger a border poll. The EU may then temporarily exempt the Irish Republic from enforcing customs and regulatory checks to allow time for the poll. More conceivable in the event of a disorderly Brexit, London, Dublin, Belfast and Brussels – and potentially Washington – could assemble in a slightly depoliticized atmosphere to determine how to safeguard the Good Friday Agreement. The likeliest solution would resemble the Northern Ireland-specific backstop that the EU initially proposed, entailing checks in the Irish Sea between goods traveling from Britain to Northern Ireland.

Whatever happens next, the Irish Republic and Northern Ireland must play their cards carefully. Thus far, all sides have prioritized political concerns over economic ones, but that may change once the economic consequences are impossible to ignore. In that case, Ireland, Northern Ireland, or both might find that they’re expendable.