Warnings about the damaging impact on the Northern Ireland peace process of the return to a physical border between the north and the south post-Brexit understate the danger. Those issuing these warnings point to the problems posed by a hard border to relations between nationalist and unionist communities, to power sharing between Sinn Fein and the Democratic Unionist Party (DUP) and to commerce within Ireland and between Britain and Ireland.

But the opposite of a peace process is a war process and this is not so far away as it might seem. Peace in Northern Ireland depends ultimately not so much on power sharing but on a complicated but stable balance of power between communities and it is this which is now being eroded by a Brexit-obsessed British Government.

A central ingredient for violence in Ireland between 1968 and the Good Friday Agreement in 1998 was that for most of that period British governments were in effect supporting the predominance of Protestants over Roman Catholics. It became more decorous to use political epithets – unionists and nationalists – to refer to the two sides, but the sectarian divide has always been at the heart of the Troubles since the first civil rights marches in 1968, complicated though the conflict has always been by the broader claims of Irish nationalism. So long as this was the British posture, the balance of power was always skewed against constitutional non-violent nationalist opposition to the status quo.

The British position changed in 1990, when Peter Brooke, the Northern Ireland Secretary, said that Britain had no bias or national interests of its own in Northern Ireland. This made the relationship between nationalists and unionists more even and opened the door for even-handed mediation by successive British and Irish governments. Compromise between unionists and nationalists became more feasible – and the use of the gun more counterproductive – once bargaining had begun about how power should be divided.

It is this process which is now going into reverse: Brexit makes Northern Ireland more distinctly British, which is why the DUP supports Britain’s departure from the EU, despite the damage to local economy. What makes the border issue so much more inflammatory than it would otherwise be is that the British Government is no longer neutral: on the contrary, its very existence depends on being supported by the votes of the DUP in Parliament.

It is extraordinary that Theresa May’s deal with the DUP after she lost her parliamentary majority in the general election in June should have gone through with so little protest or realisation of its destructive consequences for peace in Northern Ireland. It is absurd to imagine that the present British Government, wholly absorbed in negotiating Brexit and determined not to hold another general election which it would probably lose to Labour, is in any position to mediate fairly in Northern Ireland.

British commentators on “the border and Brexit” generally feel that any reference to Ireland is incomplete without a dollop of history thrown in. They frequently cite Churchill’s hackneyed line about the re-emergence of “the dreary steeples of Fermanagh and Tyrone” as an issue, unaffected by the cataclysm of the Great War. The quote has a patronising ring to it – “Why are those fellows so obsessed by the past?” – and is in any case misleading about present developments. What is really re-emerging with the Conservative-DUP alliance is a return to the fatal combination between the Conservatives and the unionists in Ireland which in the late 19th and early 20th century defeated or blocked successive Home Rule Bills. It was this failure of constitutional nationalism that gave legitimacy to physical force as an alternative option.

A further reason why the gun and the bomb may come back into Northern Ireland politics, and thus into relations between Ireland and Britain, is that they have proved effective in the past. Prior to 1968, the nationalist community in Northern Ireland suffered from being a minority discriminated against by unionist governments permanently in power and backed by the British state.

A couple of years before the violence began, a British newspaper had published an excoriating piece about the way the province was run, called “John Bull’s Political Slum”, but British governments had blithely ignored the stench from the slum since 1922. It was only when street protests morphed into violence and, soon after, into a vicious guerrilla that they began to pay attention.

Constitutional Irish nationalists are often self-deceiving or dishonest about the degree to which their own leverage has depended on the alternative to them being the gunman. I was living in Belfast during the height of the troubles between 1972 and 1975 when unionist politicians complained that the Social Democratic and Labour Party (SDLP), the main political voice of the nationalist community, depended for their political clout on the actions of the IRA. The SDLP leaders brushed this criticism aside, saying that they absolutely condemned “the men of violence”.

Their condemnation may have been sincere, but that did not mean that they did not benefit politically from the actions of the IRA. One person on the nationalist side who was realistic about this was the SDLP leader Paddy Devlin, briefly a minister in a power-sharing government in 1974. I remember him saying to me that “the unionists have a point: I can pick up a phone and get put through to almost any British minister, aside from the prime minister. I can do this not because they care very much about me or the SDLP, but because they would prefer to talk to us than to the Provisional IRA”. In due time, British governments ended up doing just that, however much they tried to disguise the fact.

ORDER IT NOW

It is depressing to see how quickly the lessons of the 30-year-long war in Ireland are being forgotten and old mistakes repeated, though half a century ago they led to the most intense guerrilla war fought in Western Europe since the Second World War. One of the few great successes of British governments in recent decades was to end the conflict which it only did after immense and sustained efforts.

This achievement is now being thrown away. The proponents of Brexit, insofar as they thought about the border issue at all, regarded it as minor one. So it is in terms of population and geographical area involved, but this did not prevent it becoming a running sore previously and there is no reason – given the British Government’s present political trajectory – that it should not do so again.

Gerry Adams and Martin McGuinness may no longer lead Sinn Fein, but this is scarcely comforting. While they were there, they carried a degree of authority and control within the Republican movement that their successors cannot match. The DUP, the party of Ian Paisley and which has a deeply sectarian tradition, has an armlock on the British Government while a reinvigorated border will make Northern Ireland more British and less Irish. It would be surprising if there are not some Republicans who think that Britain is discarding the long-negotiated agreements and compromises that brought peace. There may not be many people who think so, but then you do not need many to bring the gun back into the politics of Ireland.