On Friday, the prime minister delivered a reasonable, moderate, detailed speech, explaining the type of Brexit sought by her government.

A sizeable chunk of that address dealt with the Irish border, which has become the negotiations’ most fraught issue. Increasingly, the EU Commission uses emotive language about the Belfast Agreement and the Northern Ireland peace process to provoke the UK and probe the sorer points of British domestic politics.

Many remainers and leavers alike praised Theresa May’s constructive approach to maintaining a ‘soft border’, but her ideas are unlikely to be accepted in Dublin or Brussels. The European Parliament’s belligerent ‘Brexit coordinator’, Guy Verhofstadt, described the speech as ‘vague aspirations’, while the Irish government’s foreign minister and nationalist ideologue-in-chief, Simon Coveney, denied the prime minister’s proposals could amount to a realistic “end-point”.

The address at Mansion House was the best plan that May has set out for Brexit so far, but the government made a basic error right at the start of negotiations by ruling out a ‘hard border’ in Ireland unilaterally. That promise provided the EU with an incentive to be unreasonable. Its negotiators can fabricate a series of endless objections to achieving a ‘frictionless’ frontier and still claim with credibility that it’s Britain’s responsibility to avoid customs checks and other infrastructure.

Brussels’ strategy reached a new low last week when the Commission published its Draft Withdrawal Agreement. This document corrupted the UK’s undertaking to preserve regulatory alignment between Northern Ireland and the Republic of Ireland in a limited number of areas and turned it into a binding legal commitment that Northern Ireland should stay in the Customs Union and Single Market, even while the rest of Britain leaves.

This ‘backstop’ is an incendiary proposal, that disregards the principle that any change to the province’s constitutional status within the UK can be contemplated only “with the consent of a majority of its people”. The Commission claims that it’s concerned with maintaining the Belfast Agreement, yet it’s prepared to disregard the most basic tenet of the peace process, to make things more difficult for Britain.

In order to reach a satisfactory deal on Ireland, that protects the UK’s territorial integrity and allows Northern Ireland to leave with the rest of the country, Theresa May must be prepared to contemplate a hard border on the island. Britain has gone to extraordinary lengths to soothe the fears of Irish nationalists and the Dublin government, but that tactic on its own is only encouraging a bolder, more antagonistic mood.

Last week, the Irish prime minister, Leo Varadkar, went so far as to encourage Sinn Fein to end its policy of abstention and take its seats in the House of Commons, to “defend the interests of the island of Ireland”. The Taoiseach, who attacks the IRA’s political wing bitterly in the Republic’s domestic parliament, is urging its MPs to help frustrate Brexit at Westminster.

It’s unlikely that Dublin really wants the trouble and expense of absorbing Northern Ireland into a ‘united Ireland’ in the foreseeable future, or that Varadkar even cares that much about Irish citizens north of the border. Yet his policies are shaped by nationalist assumptions that British authority in the six counties must be curtailed and that the Republic is entitled to an ever-expanding say in the province’s internal affairs.

Brexit has inspired the Irish government to promote a type of all-island nationalism that won’t leave it with genuine responsibility for Northern Ireland or the cost of paying any bills. So long as the region remains London’s problem ultimately, why not move the border to the Irish Sea and force closer political and economic integration between north and south?

It’s an appealing alternative to Varadkar’s preferred option that the whole UK stay in the Single Market. The Republic’s economy, just like Northern Ireland’s, is far more threatened by barriers affecting east-west trade with Great Britain, or the flow of goods reaching the island from the continent, than disruption to the much smaller volume of business conducted across the land frontier.

Of course, Theresa May and the British government should emphasise that they will do everything they reasonably can to prevent a hard border in Ireland, but they mustn’t keep saying that it’s unthinkable. Currently, the EU is presenting an aggressive proposal to change the UK’s constitutional order as the only viable alternative. The prime minister has to make it clear that, while no-one wants a hard border, it’s preferable to any type of border in the Irish Sea, even one that is largely symbolic.

There’s a common assumption on the British side that, despite all the hostile megaphone diplomacy, the EU wants a stable, workable relationship with the UK after Brexit. The noisy fuss of the withdrawal negotiations will eventually have to die down and the two sides can then start the serious work of brokering a mutually beneficial deal.

That may yet be the case, but it will take much longer to reach a point of pragmatism if the Commission and Dublin feel there will be no consequences to their current behaviour. It’s fine for the UK to take the moral high-ground, but the idea that certain topics like the Irish border or the security order in Europe cannot even be put on the table is likely to be self-defeating.

Theresa May was right to mix realism and conciliation in her speech last week and that should continue to be the overarching tone from British negotiators. However, it’s important that the EU and its member states at least consider that they could fail to get what they need out of any final deal if they don’t treat the UK with consideration and respect.

As long as all parties are sensible and reasonable, there’s no need for a hard border in Ireland. But it could yet happen, particularly if the EU tries to exact revenge on the UK for Brexit or if it allows Irish nationalists to use the withdrawal negotiations to attack British sovereignty.