Opinion | Brexit, Ireland and the Failure of the European Idea

In January, just a few miles from were I met the angry local man, a car bomb exploded in the center of Derry. No one was injured, luckily, but the entire island was unnerved. The police blame a splinter group called the New I.R.A.

Though the I.R.A. is a small shell of its old self, some suspect that it is eager to see a hard border, and the strife that would accompany it. Martin McAllister, an ex-I.R.A. member I met in South Armagh, said he also quit the organization in the 1970s, when he decided it had devolved into a terrorist criminal enterprise. A few years ago, after he began speaking out about I.R.A. gangsterism, he was ambushed and beaten.

Driving through his home region, Mr. McAllister pointed out a remarkable number of oil tanks and fuel trucks — the sign, he said, of a gasoline-smuggling ring, a continuing activity left over from the Troubles.

“For the I.R.A., this was a way of making money to pursue the war,” he said. “Some of the main leadership of the I.R.A. have been personally involved in it for personal gain.” Profits are lower today, but a border will change that. “If there is a hard border, all these smugglers will have a great time.”

If you believe Mr. McAllister and Mr. O’Rawe, a hard border across the island will effectively be a tripwire. Locals will loathe it. Gangsters will love it. And ultimately, a generation of new resistance fighters might find themselves in a conflict of interest with Godfathers-turned-freedom fighters.

A whole generation of Europeans has grown up seeing Britain as a nation that overcame its bitter sectional differences — proof that cooperation worked, that borders might be a thing of the past. Now not only is Britain turning its back on Europe, but it is also turning its back on one of the signal achievements of the European idea. The ramifications of that decision will reach far beyond Derry and South Armagh.

Jochen Bittner is a political editor for the weekly newspaper Die Zeit and a contributing opinion writer.

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