On the surface, the results of the 2017 Northern Ireland assembly elections look very familiar. The largest unionist party, the DUP, has again come first in Thursday’s voting, as it has done in all five assembly elections of the 21st century. The largest nationalist party, Sinn Féin, has come second, also for the fifth time in a row. The other parties trail in the DUP’s and Sinn Féin’s wakes, again in the usual order. Heady talk that 2017 might prove to be the election in which cross-community transfers of votes might finally dent the historic sectarianism of elections has proved misplaced. The old familiarities have reasserted themselves.

The immediate consequences also have a predictable air. Under Northern Ireland’s power-sharing political settlement, the main parties must now negotiate a programme for joint government. The Northern Ireland secretary, James Brokenshire, has invited them all for talks at Stormont on Monday. But progress is unlikely to be rapid. The last power-sharing executive led by the DUP and Sinn Féin collapsed in January because the DUP leader, Arlene Foster, refused to step aside to allow an investigation of the flawed “cash for ash” renewable energy scheme, which she oversaw. That issue remains unresolved. It is therefore a serious stumbling block to a new agreement within the appointed three weeks. In Northern Ireland’s zero-sum politics, that means Mr Brokenshire may fail to broker a new deal. This could lead to direct rule from Westminster, with the assembly suspended, or to yet another election.

All this has a grim familiarity. Yet beneath the surface, the results reveal very significant shifts. The DUP may have once more emerged on the top of the pile, but its share of the first preference vote has slipped for the third election in a row, now down to 28.1%. Sinn Féin’s, in contrast, has risen to its highest ever share, 27.9%. The gap between them across the whole of Northern Ireland is now fewer than 1,200 votes, and there is only a one seat difference between them in the slimlined 90-seat assembly. For the first time in Northern Ireland’s devolved politics, the two main nationalist parties (Sinn Féin and the more moderate SDLP) now have more seats than the two main unionist parties (the DUP and the UUP, whose leader, Mike Nesbitt, has resigned). The voters have not produced the new non-sectarian result that Mr Nesbitt and some others hoped for. But they have not produced the same old result either.

The bulk of this shift has been caused by the DUP circling the wagons. Ms Foster could have avoided all this simply by standing aside for a few weeks, and in a less polarised polity she would have resigned before the old assembly collapsed. She remains a block in the road, just as before the election. Sinn Féin advanced because its campaign centred on the demand to be treated equally and with respect by unionists such as Ms Foster. That is a resonant demand in Northern Ireland. In some ways it is what the whole period of the Troubles and the peace process has been about from a nationalist standpoint. The upshot was that nationalist Northern Ireland rallied behind Sinn Féin and the SDLP last week more thoroughly than unionist Northern Ireland rallied behind the DUP and UUP. She may lead the largest party, but the logical thing is for Ms Foster to consider her position – as others are surely doing for her.

The UK’s disastrous vote for Brexit makes matters even more fraught, and the formation of a power-sharing executive even more imperative. The DUP’s decision to support leaving the EU in the 2016 referendum was massively irresponsible. It sent yet another message of disrespect to nationalists north and south. It was also out of line with Northern Irish opinion, which voted to remain. But post-unionist pro-remain voters have not emerged as a significant force from within the unionist community, so it would be both imprudent and irresponsible for Sinn Féin to push too hard its demand for a border poll. The impact on communal relations would be far more toxic than that of any assembly election.

Theresa May’s hard Brexit is irresponsible too. It makes the consequences in Ireland even worse, as a break with the single market and, in particular, the customs union, inescapably makes a hard border into a reality. That would be a disaster for both parts of Ireland, with major implications for the rest of the UK and its component parts. This is why Northern Ireland’s latest political impasse matters so much, to all of us in Britain as well as in both parts of Ireland. We cannot sit idly by as this country is broken up and sacrificed on the altar of Brexit.