f 23 out of your 27 devolved assembly members, plus four of your eight Westminster MPs, along with lots of your local councillors and party members, have all signed letters calling on you to resign, then politically speaking, you are already toast. This was Arlene Foster’s unhappy fate, duly confirmed today. For the Democratic Unionist party leader and Northern Ireland first minister, it is the end of the road. Foster’s ousting has many causes and it will have many consequences. It is also an event without local precedent. Remarkably, the DUP has never in its history had a leadership contest (nor, of course, has its rival, Sinn Féin). The DUP’s no-surrender founder, Ian Paisley, led the party into a power-sharing government without being seriously challenged. His successor, Peter Robinson, survived a marital crisis without losing his grip over his socially conservative party. Foster also toughed out the “cash for ash” renewable energy scheme scandal, which shut down the Northern Ireland assembly for three years from January 2017. So something new is happening in what can seem like the unchanging world of Northern Ireland unionism. Viewed in the round, the issue that has brought Foster down and the DUP close to internal panic is the working of the Northern Ireland Brexit protocol, which creates an Irish Sea border between the region and the rest of Britain. As well as interrupting supplies from Britain, the Irish Sea checks drive a wedge into Northern Ireland unionism’s self-identity as part of the UK. The DUP is terrified that it will lose its hegemony with unionist voters over the issue. But Foster’s position was also weakened by a certain instinct for practicality that jars with the party’s what-we-have-we-hold fundamentalists. When she is interviewed, Foster may strike many in Britain as just another hardliner. Nevertheless, she always opposed a hard Brexit, preferring “a workable plan”. She also backed attempts to make the protocol work. “We will take any benefits that flow from the protocol,” she said last December, a position she was forced to abandon as more militant protests began. Foster has been somewhat more liberal on some social issues. She has stressed that the DUP has gay members and supporters. Last week, with four other DUP members, she abstained on an assembly motion calling for a ban on “gay conversion therapies”. In a party that is still dominated by strict morally conservative Protestantism, this was a bold act. The timing of the backlash against Foster owes as much to this as to the protocol. Yet shed few tears for Foster. It is Brexit that has put the DUP on the rack. And it is Brexit that she and her party got horribly wrong from the start. Pure UK sovereignty was always going to be incompatible with the Northern Ireland peace process, into which power-sharing was hardwired. The DUP should therefore have thought the issue through and opposed Brexit, as a majority of Northern Ireland voters unsurprisingly did in 2016. When the UK nevertheless voted for it, the DUP should have backed Theresa May’s clunky but, in this context, principled Northern Ireland backstop. Instead, the DUP foolishly threw in its lot with Boris Johnson and the European Research Group. Three years ago, Johnson went to the DUP conference and promised “no British government could or should” sign up to a border in the Irish Sea, a pledge he repeated when he became prime minister in 2019. The following year, Johnson signed up to precisely that, making idiots of the DUP. Johnson has no interest in Northern Ireland. A colleague from his Daily Telegraph days recently recalled him announcing he was writing a piece about Northern Ireland. “Remind me,” he asked, “which ones are the orange johnnies?” Foster is neither the first nor the last person to make the error of believing Johnson’s lies. But she is learning her lesson now. She is also being made the scapegoat for wider DUP and unionist failings that will continue to confront her successor. A hundred years ago next week, Ireland was partitioned. The anniversary looms large in Northern Ireland, fairly large in the republic, and not at all in Britain. This reflects the reality, expressed by Philip Kerr, David Lloyd George’s private secretary and one of the architects of partition in 1921, that the new border was intended to “take Ulster out of the Irish question” and at the same time to “take Ireland out of English (sic) party controversies”. The penny seems finally to have dropped on the limits of British political engagement. A recent Queen’s University survey found just 5% of Northern Ireland voters trusted the UK government over the protocol. They are wise to be sceptical, but the implications are disturbing. Johnson’s pretence that there is no border in the Irish Sea is not just untrue but dangerous. Even under Foster, the DUP struggled to make an effective political strategy out of these truths. Confronted with difficulty, the DUP is always tempted to fall back on the past. But a combination of demographics, greater prosperity, cultural change and the bleak reality that a return to sectarian violence always threatens continues to push Northern Ireland slowly and pragmatically towards the future. The DUP desperately needs a clearer vision of where Northern Ireland unionism’s own best interests lie. But it also needs a British government that is readier to support that vision, not to play games with it. Martin Kettle is a Guardian columnist Join a Guardian Live discussion on the growing tensions in Northern Ireland in a livestreamed event, with the Guardian’s Rory Carroll and Lisa O’Carroll, the DUP’s Carla Lockhart and the Alliance party’s Naomi Long. Thursday 13 May, 7pm BST (8pm CEST, 11am PDT, 2pm EDT). Book tickets here