On 8 May 1987 a Provisional IRA unit of eight men attacked a police station in the village of Loughgall in county Armagh 15 miles from the Irish border. One man drove a digger with a bomb in its bucket towards the building, half of which was destroyed in the explosion. But British forces had been informed of the time and place of the assault and SAS soldiers waiting in ambush opened fire killing all eight Provisionals and a civilian.

A quarter of a century later in county Monaghan just inside the border with the Irish Republic but not far from Loughgall, there was an incident proving that the earlier killings were still a live issue. In the last few days somebody, evidently an opponent of the IRA, used a bulldozer to demolish a substantial memorial to two IRA men, Jim Lynagh and Padraig McKearney, who had died in the SAS ambush.

A statement from the Loughgall Truth and Justice Campaign described the bulldozing of the memorial as a “desecration” and declared that “to do this to any of the Loughgall families is to do this to us all... but our memories and thoughts cannot be erased”.

The episode is significant because it shows the human and divisive reality of the Irish border and why its reappearance at the top of the political agenda is such a threat to long-term peace. The backstop is often discussed in Britain as if it was an issue primarily to do with trade which has been given exaggerated significance by Ireland and the EU in order to sabotage Brexit. Boris Johnson denounces it as being unacceptably “anti-democratic”.

In all cases, there is blindness towards the true reason for the toxicity of the dispute over the 310-mile border which stems from it being the physical embodiment of relations between nationalists and unionists, Catholics and Protestants not just in the border region but in the north as a whole. That is why it has been one of the most fought-over and blood-soaked frontiers in Europe over the last 400 years. The map of the area is dotted with the names of battles ancient and modern. The destruction of the Loughgall monument shows that antagonisms have not moderated and, while some people feel strongly enough to build a memorial to two dead IRA men, others feel strongly enough to destroy it.

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The visit of Boris Johnson to Belfast this week reveals once again the mixture of frivolity and ignorance with which the Brexiteers approach Northern Ireland. A new post-Brexit border is supposed to be monitored remotely by yet-to-be discovered technical means. But it should be self-evident that any CCTV or other gadget located on the border in a nationalist/Catholic area will be torn down in a few minutes.

The neutrality of the British government between nationalists and unionists was the foundation of the Good Friday Agreement of 1998 that ended thirty years of war in Northern Ireland in which two per cent of the population was killed or injured according to historians of the conflict (the same proportion of casualties in Britain as a whole would have meant 100,000 dead).

Careless of this sanguinary record, Johnson’s approach is entirely opportunistic: he will maintain UK neutrality but he expresses an undying commitment to the union. He and the new minister for Northern Ireland had a convivial dinner with the DUP leader Arlene Foster, on whom the Conservatives depend for their majority, before meeting the leaders of other parties. DUP activists make clear in private that they would like a hard Brexit regardless of economic cost because they want to keep as far from the Irish Republic and as close to Britain as possible.

Supporters of the Good Friday Agreement (GFA) comfort themselves by saying that the Conservatives kowtowing to the DUP will last only as long as they rely on DUP votes in parliament. This could prove over-optimistic: Johnson leads a hard-right government riding a resurgent wave of English nationalism in which anti-Irish sentiment has always had an integral part.

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This does not mean that a shooting war is going to restart any time soon. The unreconciled fragments of the IRA are disorganised and lack popular support. But the building blocks of the GFA are being kicked away one-by-one. The power-sharing executive and Northern Ireland Assembly are suspended and are unlikely to be resurrected.

The DUP understandably prefers to share power with the Conservatives in Westminster than with Sinn Fein in Belfast. Sinn Fein, for its part, does not want to be the junior and largely impotent partner of the DUP in an executive which would be complicit in implementing a no-deal Brexit which it opposes.

Sinn Fein can also see a substantial silver lining for its brand of Irish nationalism in the present crisis. Northern Ireland voted 56 to 44 per cent to stay in the EU and, when the Conservatives ignore this and pretend that the DUP’s pro-Brexit stance represents majority opinion in the province, they de-legitimise the union with Britain. This will not necessarily impel pro-Remain unionists to vote for a united Ireland, but it does mean that the significant minority of Catholics/nationalists who previously preferred to stick with the union is fast diminishing.

This will matter because in the not-too-distant future Catholic voters will outnumber Protestants in Northern Ireland. The outcome of a border poll will become more incalculable. But even the prospect of one – strongly advocated by Sinn Fein – will be deeply polarising. Brexit has succeeded in putting Irish Partition back at the centre of the political agenda, something that Sinn Fein had failed to do despite decades of effort.

Does this mean that Irish unity is getting closer? This prospect is increasingly if naively raised in the British media. But demographic and diplomatic change will not be sufficient in themselves to transform the political balance of power: the unionists/Protestants could not ultimately maintain their rule in the north despite being the majority. Catholics and nationalists are unlikely to be any more successful against resistance to a united Ireland by a determined Protestant minority.

Possibly Johnson’s gamble on threatening the EU states with a no-deal Brexit will pay off. They have hitherto never believed that Britain would do anything so self-destructive and they might just look to some last-minute deal. But, even if Leo Varadkar did want such an agreement, he would find it difficult to sell to Irish voters, while the EU would be seriously weakened by caving-in to Johnson’s bombast after declaring for so long that it would do no such thing.

Ireland does not relish a confrontation with the UK, but it has little choice but to demand that the EU stick to its commitments and, on the other side of the Atlantic, energise the political influence of the Irish-American diaspora. The Clinton administration was an essential driving force for the GFA. The US speaker of the house Nancy Pelosi has repeatedly said that she will block any Anglo-American trade deal if it creates a hard border or the GFA is imperilled.