On Wednesday, when Westminster was turning spectacular disarray up to 11, Arlene Foster, leader of the Democratic Unionist party, was in two places at the same time. One was Washington DC. The other was a sea of tranquillity. Her message was that no doubt the universe was unfolding as it should: “Brexit is only two weeks away. When you come to the end of a negotiation, that’s when you really start to see the whites of people’s eyes and you get to the point of a deal.” All good, then – except that something seemed a little awry with her allusions.

She was thinking of the well-known instruction: “Don’t shoot until you see the whites of their eyes.” Shooting people is not exactly the same as negotiating with them. Moreover, the phrase comes from the battle of Bunker Hill at the start of the American war of independence in 1775. It was issued by one of the American commanders – the people about to be shot were the British. And Bunker Hill was a famously pyrrhic victory for the British, after which General Henry Clinton remarked in his diary that “A few more such victories would have shortly put an end to British dominion in America”.

So maybe Foster’s allusion was not so inapt after all. For one of the reasons why British politics is in disarray is her own party’s series of pyrrhic victories. It has been in one sense a time of extraordinary and highly improbable triumph for the DUP. Here it is, a marginal party in UK terms and one that does not even represent the views of most people in Northern Ireland, acting as the final arbiter of Brexit itself. It commands not just its own 10 MPs who keep the Conservatives in power but, through its alliance with the Rees-Moggites, an effective veto over the entire withdrawal agreement. For a fervently pro-British party, this is a like a superfan being able to tell her beloved band what songs to sing and what clothes to wear.

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Yet the reality, to adapt General Clinton, is that a few more such victories will put an end to the very British dominion in Ireland that the DUP exists to uphold. It is not just that the British state itself is being so badly discredited by the goings-on in Westminster, though that is certainly part of it. The union, especially in the contested territory of Northern Ireland, depends on the prestige of Britishness. It can no longer be enforced by hard power, so it must rely on soft power. Does anyone really believe that a parliamentary game of charades in which the answer is anarchy in the UK is a good advertisement for British prestige? Oscar Wilde wrote that each man kills the thing he loves – the DUP, for all its history of homophobia, has been in this respect positively Wildean.

But at a more fundamental level, we must remember that this whole mess is rooted in the so-called Irish backstop, and that the backstop – in its present, deeply contentious, form – is a creation of the very party that now opposes it: the DUP. The crucial time was the first week in December 2017. Theresa May agreed with Michel Barnier the text of a backstop that applied only to Northern Ireland. This dealt with Irish concerns but left Britain free to pursue whatever future relationship with the EU it wanted. May was called out of the meeting to take a call from Foster, who instructed her to rescind the deal. A few days later, the Irish backstop became the UK-wide backstop. The consequences of that pyrrhic victory are now playing out in the disorder that threatens the very viability of the United Kingdom. If it is killed off, it will be, as Foster’s mangled metaphor accidentally hinted, by friendly fire.

Fintan O’Toole is a columnist with the Irish Times and author of Heroic Failure: Brexit and the Politics of Pain