On July 7, 1972, the leadership of the IRA met with the Secretary of State for Northern Ireland, William Whitelaw, in a secret meeting in London to begin negotiations on a British withdrawal. At least that is how some of the IRA men saw it.
Sean MacStiofain is said to have been miffed that they were not offered a conference table to sit at. He saw himself as resuming the unfinished business in which Michael Collins had made the mistake of conceding the partition of Ireland.
Whitelaw thought the IRA men were absurd in their demands. He had just refused an Irish government demand that he set the goal of a united Ireland as the object of British policy and he was hardly going to concede more than that to a group of people who had no mandate from anyone.
Now, you would expect that a revolutionary army which had been spurned in such a way by the enemy would return to the people it represented and tell them what had happened and explain that the rejection of their demands left them no option but to return to war. The IRA didn't do that.
Instead, it engineered a confrontation with the Army in Lenadoon on a horrific day, which started with loyalist and republican sectarian murders and ended with Army snipers shooting dead six civilians in Springhill.
The curious fact about the IRA is that it never, at that time, presented the demand for a united Ireland as the justification for violence.
It preferred to frame its objective as the defence of the Catholic people, while, absurdly, endangering those people through the bombing campaign and gun battles on housing estates.
I have spoken, as a journalist, to many members of the IRA down the years and the most common reason they give for joining and putting themselves under orders is they felt the need to strike back at the RUC and the Army.
Over and over again they say that their political education came later. Something similar is true of loyalists, too.
The IRA was less inclined to own up to its sectarian impulses, but the killing did not stop during the 1972 ceasefire. It merely diverted onto civilians; mostly, but not exclusively, Protestants. It appears that the IRA in the early days of the Troubles was not confident that its support base in the Catholic community would endorse, or even tolerate, an armed campaign for a united Ireland. There had to be a better, more immediate reason for it.
People would be more sympathetic to retaliatory strikes against the Army; and this at a time when the Army was easily provoked and inept and included in its ranks people who were just as eager for war as the republicans and loyalists were.
Now, there has been much discussion this week on the threat that a bad Brexit poses to the Good Friday Agreement. The supposition of many in the United States and in the British media and among former British prime ministers who helped frame it is that that agreement is fundamentally a peace treaty. And, logically, if a peace treaty is broken, war resumes.
Henry McDonald has been attacking the idea that our relations with the European Union have anything to do with the Good Friday Agreement. He refers to a blog post by Ed Moloney, which makes a point similar to the one I have made above; that the street fighters of Ballymurphy will take no notice of whether or not lorry drivers at the border have more documents to fill in.
You could add to that argument the fact that the power-sharing Executive collapsed for three years without "war" resuming. Then the core dispute was over an Irish language act. Now there is no act and no crisis arising from its absence either. It's remarkable how the things people get worked up about simply go away sometimes.
But, to cast our minds back to 1972 again (can't you tell that I am writing a book about 1972?), at that time, Britain and Ireland were both preparing to join the European Economic Community. They both wanted an end to the violence, because they hoped to have a creative and fruitful relationship inside the EEC and they both anticipated that membership of the EEC would ultimately resolve the whole Irish problem.
The border would disappear. There would be free movement of labour. There would be a lot less to complain about, if the real irritant was the border itself.
Ireland was already involved in discussing solutions. The Taoiseach, Jack Lynch, had had two summit meetings in September 1971 with the Prime Minister, Ted Heath, yet previously, when Patrick Hillery (as Minister of External Affairs) had raised Irish issues with the British, he had been told that they were none of his business.
See the difference? The idea of power-sharing and a place at the table for the Irish government came with shared EEC membership and was unthinkable before it. The exclusion of the Irish after EEC membership was also unthinkable.
So, I don't accept Henry's argument that the EU has nothing to do with the Good Friday Agreement. But neither do I think that the agreement will collapse with a bad Brexit, or that its collapse will lead inevitably to the men of Ballymurphy anguishing over the state of the border and leaping from their beds to take up arms.
There are, indeed, some republicans for whom the faith in a united Ireland was a driving motivation. There are the core families who preserved the republican tradition and others whose analysis, ultimately, was that only Irish unity could settle the recurring sectarian eruptions that plagued us.
Not all of them became dissidents. Some felt betrayed by the compromises made by the Adams leadership in the GFA, but do not argue for a resumed campaign.
And this is mostly because they see no possibility of a groundswell of support for such a campaign, or any prospect of it succeeding without such support, a conclusion the Official IRA reached also in 1972.
Does that mean we needn't worry about the effects of Brexit on a divided Ireland? I don't think that at all.
But I think the issues are always contemporaneous rather than historical and that bad government in Britain, strained relations between Britain and Ireland and an independent Scotland bring the question of Irish unity back to the centre of our concerns, in a way it never was during the Troubles. What we are facing into now is new.