There is no definition of good neighbourliness in foreign affairs. Alliances are fixed by treaties. Regional trade is measured in goods and services. But those things cannot describe the texture of relations between countries, the way adjacent nations rub along together.

This quality is as important to Brexit as the technical hooks on which negotiations are currently snagged. No European Union member state wanted the UK to leave, and it is hard for them not to feel aggrieved by Britain’s choices. Theresa May urges Brussels not to take offence. Less emollient leavers say the continentals should get over it and focus on mutual trade (as if their own campaign was some case study in cool rationality).

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But in Westminster, Brexit does not often look like a foreign policy issue at all. It doesn’t even look much like an effort to weigh national interests. It is a Tory fight club, a rolling bout of ego wrestling among cabinet ministers slamming each other on to the faded canvas of Britain’s reputation as a serious country. Michael Gove and Boris Johnson send menacing missives to No 10 demanding a purge of Brexit dissent in the cabinet. David Davis lets it be known he is “furious” at their meddling. The meddlers respond, via anonymous allies, that Davis is “a fuckwit”. Classy.

They all act as if Brexit is something the Conservative party will claim from Brussels and bestow on a grateful nation. They do not appear to recognise that the gift is not theirs alone to give. It will be shaped by the generosity of the other side in the negotiation. That goodwill was depleted from the start.

Before a penny of Britain’s EU budget contribution has been recouped, the decision to leave the club inflicts costs on its members. It is a tax on their economic stability and diplomatic cohesion. May insists her intent is benign, but the process itself damages everyone. Those closest to the source of grief are hurt most.

The biggest loser by a mile is Ireland. In March, the European parliament published an assessment of Brexit’s impact on EU states. “The most striking result is that Ireland suffers the same magnitude of losses as does the UK,” the authors note. This was true in optimistic and pessimistic scenarios. Whichever way you slice it, Brexit looks like economic aggression across the Irish sea.

The pain starts at the border. There are 275 crossing points on the boundary between north and south, traversed 110 million times per year. Business supply chains weave in and out of the republic. More precious than commerce is the current invisibility of a line that was so recently inked in blood. Those who patrolled it were targeted by terrorists. Many more peaceful borders are the scars of old wars, and Northern Ireland’s schisms are fresh in folk memory. A lucky generation has grown up under the shelter of the Good Friday agreement, but their parents know what violence led them there.

It is in nobody’s interests for a healing wound to be undressed. But that is where determination to leave the single market and customs union leads. Removing Northern Ireland from those arrangements forces the Republic to police what would become an external boundary of the EU – to verify that incoming goods meet the requisite standards. No one who has examined how this might be done thinks it can be achieved without friction, some roadside infrastructure and smuggling by organised crime gangs.

Yes, Gove, Johnson, Davis and May, they can see you. Europe is watching your absurd, panic-stricken squabbles

Davis recently told parliament that he was “pretty much absolutely” committed to an invisible border. In slippery Brexiter code, that means not fully committed. The logic of a hard Brexit is implacable: there will be a border. It can be on the island of Ireland or in the Irish Sea, with special customs status for the north. May must choose. For the Democratic Unionist party, from whom the Tories bought a parliamentary majority in June, the maritime option is a nonstarter. It looks like economic partition of the UK.

The Northern Ireland problem is written into Michel Barnier’s negotiating mandate as one of the three issues to be resolved before talks can progress on to the UK’s final status deal. (The other two are expat citizens’ rights and budget obligations.) That inclusion reflects Ireland’s economic vulnerabilities but also legal and moral obligations, poorly understood in London, that the EU accepts as a co-sponsor of the Good Friday agreement. Promoting peace and security by the dilution of borders is a foundational principle of the whole European project.

So Irish leverage over Brexit terms is at its high point right now, when there is capacity to obstruct progress to the next phase. As a leaked European commission document showed last week, Dublin is applying that pressure, calling for Britain to stop waffling around its open border commitments. The UK has not budged. Unnamed ministers told the Sun that the taoiseach, Leo Varadkar, is yielding to unreasonable demands by Sinn Féin’s Gerry Adams.

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At best, that was failure to imagine a prime minister acting on a reasonable evaluation of the options in a process of existential importance to his country. (With May in Downing Street maybe ministers have forgotten what that looks like.) At worst, it was a malicious effort to poison perceptions of Irish motive by indirect association with the IRA.

Either way, it exhibited the common parochial weakness of British politicians who forget that people outside the UK read English. I was in Ireland last week, and can confirm that the Sinn Féin allusion was noticed and its insidious inference understood. It was surely picked up in Brussels, too. Polyglot officials monitor the unhinged tone of UK tabloids and observe its transmission into government policy.

Yes, Gove, Johnson, Davis and May – they can see you. The rest of Europe is watching your absurd, panic-stricken squabbles and listening to your bluster. They notice how oblivious you are to the consequences of your actions for countries that once counted as your friends. They form judgments on the character of the regime with which they are dealing: its reliability, its sense of responsibility. And this affects the talks. They see a country fast degenerating from trusted ally to nightmare neighbour.

• Rafael Behr is a Guardian columnist