In january 1972, when I was a 13-year-old boy in Dublin, my father came home from work and told us to prepare for civil war. He was not a bloodthirsty zealot, nor was he given to hysterical outbursts. He was calm and rueful, but also grimly certain: Civil war was coming to Ireland, whether we wanted it or not. He and my brother, who was 16, and I, when I got older, would all be up in Northern Ireland with guns, fighting for the Catholics against the Protestants.

What made him so sure of our fate was that the British army’s parachute regiment had opened fire on the streets of Derry, after an illegal but essentially peaceful civil-rights march. Troops killed 13 unarmed people, mortally wounded another, and shot more than a dozen others. Intercommunal violence had been gradually escalating, but this seemed to be a tipping point. There were just two sides now, and we all would have to pick one. It was them or us.

The conditions for civil war did indeed seem to exist at that moment. Northern Irish society had become viciously polarized between one tribe that felt itself to have suffered oppression and another one fearful that the loss of its power and privilege would lead to annihilation by its ancient enemies. Both sides had long-established traditions of paramilitary violence. The state—in this case both the local Protestant-dominated administration in Belfast and the British government in London—was not only unable to stop the meltdown into anarchy; it was, as the massacre in Derry proved, joining in.

Yet my father’s fears were not fulfilled. There was a horrible, 30-year conflict that brought death to thousands and varying degrees of misery to millions. There was terrible cruelty and abysmal atrocity. There were decades of despair in which it seemed impossible that a polity that had imploded could ever be rebuilt. But the conflict never did rise to the level of civil war.

However, the belief that there was going to be a civil war in Ireland made everything worse. Once that idea takes hold, it has a force of its own. The demagogues warn that the other side is mobilizing. They are coming for us. Not only do we have to defend ourselves, but we have to deny them the advantage of making the first move. The logic of the preemptive strike sets in: Do it to them before they do it to you. The other side, of course, is thinking the same thing. That year, 1972, was one of the most murderous in Northern Ireland precisely because this doomsday mentality was shared by ordinary, rational people like my father. Premonitions of civil war served not as portents to be heeded, but as a warrant for carnage.