But, as anyone who goes to the border discovers, you can’t ignore the past. Talk to people in Northern Ireland, and they’ll bring up events from decades, or even centuries, ago, with a visceral emotion that would make you think they’d happened just last week. Investigators are still churning up the soil, searching for the bodies of people who were murdered during the Troubles and buried in secret graves.

Make no mistake: The border is a scar.

And the scar is about to open. Brexit has foundered, in no small measure, on the question of how to cope with the Irish border. Prime Minister Theresa May has promised future technological innovations that would allow for a frictionless customs border, but for the moment, that is magical thinking. Under the withdrawal agreement rejected for a third time on Friday both the United Kingdom and the European Union are committed to a “backstop,” which, in the event that they have failed to negotiate a future relationship by the end of 2020, would prevent the abrupt reintroduction of a hard border. But that is, at best, a temporary measure.

Will the violence return? It already has. In January, a car bomb exploded in Derry. This month, a dissident group claimed responsibility for several unexploded letter bombs that were sent from Ireland to London and Glasgow.

I don’t believe that we will see a return to the full-blown violence of the Troubles. The paramilitary groups that oppose the Good Friday Agreement are marginal players, with none of the resources or popular support that empowered the I.R.A. and others. But then, conflicts seldom start because most of the population wants them to.

The Troubles ignited in the first place, in large measure, because limited acts of violence were met with irrational escalation. Station a customs man at a border crossing, and kids are going to throw rocks at him. So you bring in the cops to stand guard. Then some fool with a rifle shoots a cop. What happens next?

One solution, which the European Union proposed last year, was to shift the border, for customs purposes, into the Irish Sea, so Northern Ireland and the Republic could trade without barriers. The British government rejected this idea on the grounds that doing so would undermine the unity of the United Kingdom. It’s a further measure of the Brexiteers’ naïveté that they don’t realize that by forcing Northern Ireland to choose between the United Kingdom and Europe, they may have inadvertently hastened the eventual reunification of Ireland.

Some prominent Irish Republicans have already called for a referendum on the question of Irish unity. How strange to think it could be Brexit that finally gets the British out of Ireland — an outcome that three decades of appalling violence failed to achieve.