The frustrating thing about the row over the impact of Brexit on the Irish border is that there is an obvious and straightforward solution. When Britain eventually leaves the EU, Northern Ireland should remain within the customs union.

While not perfect, it is undeniably the simplest way to avoid the difficulties that would come with a hard border between the two parts of Ireland. But this proposal has been rejected by the DUP. Yet it is tough to be sympathetic to their stance, because ultimately it is one based exclusively on symbolism rather than pragmatism.

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DUP MP Sammy Wilson stated recently that if the Conservatives are prepared “to have Northern Ireland treated differently than the rest of the UK”, his party could no longer support the government.

The problem with this logic is that since the formation of the United Kingdom in 1707, there has always been considerable differentiation between its constituent parts. Scotland had the Presbyterian kirk as its official state church while England had an Anglican equivalent. An independent and separate Scottish legal system is still in place today.

Nor is Northern Ireland any different in this regard. Northern Irish banknotes are not accepted in Britain. Gay marriage and abortion are legal in Britain, not so in Northern Ireland. Indeed unionists over the last century have shown themselves to be quite willing to accept different treatment from the rest of the UK when it suits their own predilections.

Some would argue that the key to the union enduring over the last 300 years has been the willingness to show administrative flexibility toward local particularisms. Northern Ireland remaining in the customs union would not change that.

It is hard to see what practical difficulties the DUP see with this proposal. Granted, the need to inspect trade goods entering Britain from Northern Ireland could conceivably add something to their cost. But would this really not be compensated for by Northern Ireland retaining free access to the EU markets?

Such considerations appear pointless, however. The DUP position is unassailable. Through their “confidence and supply” arrangement with the Tories, Arlene Foster and colleagues can block any move to keep Northern Ireland in the customs union.

The Tories potentially have a howitzer to deploy against their allies. And that is a border poll

Or so we are told. But the reality is different. If the DUP feels its “confidence and supply” arrangement serves as a big stick to keep the Tories on their side, then they fail to realise that the Conservatives potentially have a howitzer to deploy against their government allies. And that is a border poll.

Under the Good Friday agreement, a border poll can only be called by the secretary of state for Northern Ireland. The agreement says the secretary can order such a vote “if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should … form part of a united Ireland”.

This language is intentionally vague. The current secretary is James Brokenshire, a Conservative MP. If he decided to initiate a border poll tomorrow, there is nothing the DUP could do to stop him.

The DUP might not like the idea of remaining in the EU customs union, but it would look a great deal more appealing if the alternative was dealing with a border poll.

In the 2016 Northern Irish assembly elections, 46% of first-preference votes went to parties opposed to Irish unity, with 40% for those in favour. That same year, 56% of the Northern Irish vote was cast against leaving the EU. A vote to retain the status quo would probably begin as the favourite, but a unionist victory in a border poll would be far from a certain thing.

In other words, if DUP resistance is the only thing stymying the best solution to the Irish border issue, then the Tories have ample ammunition to play hardball.

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Except, of course, that this has ideological problems for them as well. John Major’s 1993 declaration that the British government has no “selfish interest” in keeping Northern Ireland within the UK probably reflects the attitude of the majority of British people. But not that of hard Brexiteers.

They suffer from what JA Hobson once termed “milomania”, the desire to hoard land on a map. They could no more stomach a border poll than the DUP, afraid of the vote going the “wrong” way. After all, the worst way to launch empire 2.0 would be to surrender the final original piece of empire 1.0.

Thus, ideological blindness stops the DUP from accepting a reasonable solution and stops the Tories using their obvious leverage to break the impasse. The irony is that, for over two centuries, the political difficulties between Britain and Ireland have been largely attributed to irrational Irish intransigence in response to enlightened and practical British benevolence. How times have changed.

• Caoimhín de Barra is assistant professor of history at Drew University in New Jersey