I was sitting in a cafe on the Falls Road in heavily nationalist West Belfast when a local radio reporter came in looking for residents to interview about the effect of Brexit on Northern Ireland. She said that the impact was already massive, adding: “Stupid, stupid English for getting us into this pickle. We were doing nicely and then they surpassed themselves [in stupidity].”

It does not take long talking to people in Northern Ireland to understand that almost everything said by politicians and commentators in London about the “backstop” is based on a dangerous degree of ignorance and wishful thinking about the real political situation on the ground here. Given how central this issue is to the future of the UK, it is extraordinary how it is debated with only minimal knowledge of the real forces involved.

The most important of these risks can be swiftly spelled out. Focus is often placed on the sheer difficulty of policing the 310-mile border between Northern Ireland and the Republic of Ireland because there are at least 300 major and minor crossing points. But the real problem is not geographic or military but political and demographic because almost all the border runs through country where Catholics greatly outnumber Protestants. The Catholics will not accept, and are in a position to prevent, a hard border unless it is defended permanently by several thousand British troops in fortified positions.

The threat to peace is often seen as coming from dissident Republicans, a small and fragmented band with little support, who might shoot a policeman or a customs’ official. But this is not the greatest danger, or at least not yet, because it is much more likely that spontaneous but sustained protests would prevent any attempt to recreate an international frontier between Northern Ireland and the Republic that wasn’t backed by overwhelming armed force.

It is unrealistic to the point of absurdity to imagine that technical means on the border could substitute for customs personnel because cameras and other devices would be immediately destroyed by local people. A new border would have to be manned by customs officials, but these would not go there unless they were protected by police and the police could not operate without British Army protection. Protesters would be killed or injured and we would spiral back into violence.

Britain Before Brexit: Northern Ireland Show all 12 1 /12 Britain Before Brexit: Northern Ireland Britain Before Brexit: Northern Ireland Derry, Londonderry A garage door displaying unionism, bolted shut, like a visual representation of Brexit Britain, locked to outsiders, safeguarding what’s inside Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry Rossville Street, the site of Bloody Sunday, where messages demand a severance with England. From this perspective, Britain is England in sheep’s clothing, the real empire, the centre of colonial power Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Bangor A political message in paint not yet dry, still forming, setting, adjusting, or in old paint finally eroding, melting away Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Bangor Moral judgement frames a residential view. The message seeks to make everybody involved in the religious narrative: those who don’t believe are those most in debt Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Castlerock The beach is sparse and almost empty, but covered in footprints. The shower is designed to wash off sand, and a mysterious border cuts a divide through the same sand Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast Two attempts to affect and care for the body. One stimulated by vanity and social norms and narratives of beauty, the other by a need to keep warm in the winter night Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast The gate to an unclaimed piece of land, where nothing is being built, where no project is in the making, where a sign demands the creation of something new Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry Under a motorway bridge a woman’s face stares, auburn and red-lipped, her skin tattooed with support for the IRA and a message of hostility to advocates of the Social Investment Fund Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry The Fountain Murals, where the curbs and the lampposts are painted the red, white, and blue of the Union Flag. A boy walks past in the same colours, fitting the scene, camouflaged Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Coleraine A public slandering by the football fields, for all to see or ignore. I wonder if it’s for the police or for the community Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Belfast A tattoo parlour, where the artist has downed tools, momentarily, bringing poise to the scene, which looks like a place of mourning, not a site of creation Richard Morgan/The Independent Britain Before Brexit: Northern Ireland Derry, Londonderry A barrier of grey protects the contents of this shop, guarding it from the streets outside, but it cannot conceal it completely, and the colours of lust and desire and temptation cut through Richard Morgan/The Independent

We are not looking at a worst-case scenario but an inevitability if a hard border returns as it will, if there is a full Brexit. The EU could never agree to a deal – and would be signing its own death warrant if it did – in which the customs union and the single market have a large unguarded hole in their tariff and regulatory walls.

An essential point to grasp is that the British government does not physically control the territory, mostly populated by nationalists, through which the border runs. It could only reassert that control by force which would mean a return to the situation during the Troubles, between 1968 and 1998, when many of the 270 public roads crossing the border were blocked by obstacles or cratered with explosives by the British Army. Even then British soldiers could only move through places like South Armagh using helicopters.

The focus for the security forces in Northern Ireland is on dissident Republican groups that never accepted the Good Friday Agreement. These have failed to gain traction inside the Roman Catholic/nationalist community which has no desire to go back to war and give up the very real advantages that it has drawn from the long peace.

But that peace could slip away without anybody wanting it to go because Brexit, as conceived by the European Research Group and as delineated by Theresa May’s red lines, is a torpedo aimed directly at the heart of the Good Friday Agreement. This meant that those who saw themselves as Irish (essentially the Catholics) and those who saw themselves as British (the Protestants) could live peacefully in the same place. Moreover, the agreement established and institutionalised a complicated balance of power between the two communities in which the Irish government and the EU played a central role.

Yet ever since the general election of 2017, when May became dependent on the Democratic Unionist Party (DUP), it is the DUP – the party of Ian Paisley – that has been treated by politicians and media in Britain as if they were the sole representatives of the 1.9 million people living in Northern Ireland. Its MPs are seldom asked by interviewers to justify their support for the UK leaving the EU when Northern Ireland voted for Remain in the referendum by 56 per cent to 44 per cent.

In ignoring the nationalist community in Northern Ireland, the British government is committing the same costly mistake it committed in the 50 years before 1968 which led to the fiercest guerrilla conflict in western Europe since the Second World War. The nationalist community today has a lot more to lose than it did half a century ago. It is no longer subject to sectarian discrimination in the way it used to be, as well as being highly educated and economically dynamic, but this does not mean that it can be taken for granted.

It may also be that the majority of the Northern Ireland population in two years’ time, when the Brexit transition period might be coming to an end, will no longer be Protestant and unionist but Catholic and nationalist. In the last census in 2011 Protestants were 48 per cent of the population and Catholics 45 per cent. The Protestants are not only a declining proportion of the population, but an increasingly ageing one, figures from 2016 showing that Catholics are 44 per cent of the working population and Protestants 40 per cent. Significantly, Catholics make up 51 per cent of school children in Northern Ireland and Protestants only 37 per cent.

The Protestants are a community on the retreat, but many have argued that this does not make much political difference because it is a mistake to imagine that all Catholics wanted a united Ireland. Many felt that they were better off where they were with a free NHS and an annual UK subsidy of £11bn.

But Brexit has changed this calculation. With Ireland and the UK members of the EU, religious and national loyalties were blurred. Many Protestants, particularly middle class ones, voted Remain in the referendum, but the vote was still essentially along sectarian lines. “You would not find many nationalists post-Brexit who would not vote for a united Ireland in a new border poll whatever they thought before,” said one commentator, though the likelihood is that if there were to be such a poll there would still be a slim majority favouring the union with Great Britain.