CROSSMAGLEN, Northern Ireland — You can’t see the border. It is, like the men who fought a war long over it, a ghost.

When you cross from the Republic of Ireland into Northern Ireland, the only way you know you have done so is because the service on your cellphone switches from an Irish provider to a British one.

We all thought “the border,’’ which led people to kill each other for most of the last century, was dead, dead in the grave with the sort of nationalistic fervor, both Irish and British, that engendered, encouraged, and excused barbarism.

We thought the war in Ireland was over, but 20 years after the Good Friday Agreement all but ended The Troubles, which the Irish, in their penchant for understatement call their civil war, the grim prospect of some kind of border is back, looming like the mist that rises over the hills of Fermanagh with the dawn.

When the British electorate, but only 44 percent of voters in Northern Ireland, voted to leave the European Union, they unwittingly voted to undermine some three decades of painstaking diplomacy that crafted a marvelous compromise called the Good Friday Agreement, which let everyone win by letting no one win.

Now, with Brexit in the offing, the prospect of the old customs checkpoints springing up again along the Irish border — the Republic remaining in the EU, the North exiting with the rest of Britain — threatens to slow the march toward normalcy of the last 20 years.

Even if by some political fudge the checkpoints don’t return, the anxiety over Brexit has exposed the fragility of the peace: The absence of violence in Northern Ireland has not been accompanied by widespread reconciliation or integration. Most people continue to live, go to school, and socialize with those of their same religion and/or national allegiance: Protestant unionists and Catholic nationalists remain a people largely apart.

True, they rarely kill each other now. Given that an average of 100 people were being killed each year in a place roughly the size of Connecticut, the fact that 2,000 people who would have been in the ground are walking around today is not to be dismissed.

But, in a place once so obsessed with the past, what about the future?

The view ahead is obstructed. There are still so many reminders of the past, as if it isn’t past. The way politicians talk past, not to, each other. The way most people still live in neighborhoods which are either predominantly Catholic and nationalist or Protestant and unionist. The economy of the North is still as dependent on the British exchequer as it was 20 years ago. Neither reconciliation nor private industry has grown with the peace.

Even if you don’t notice when you pass the now-invisible border, it’s impossible not to notice, and wonder at, the diesel stations on the road that runs into South Armagh off the M1 highway linking north and south. On a winding, bumpy section of road that defies the stereotype of the roads in the north of Ireland being inherently better than those in the south, there are no fewer than eight diesel stations in a very sparsely populated area.

The locals say even the dogs in the street know many if not most of these diesel stations are controlled by what was the Republican movement’s lucrative smuggling industry. Southerners still drive over the border to buy the cheaper diesel.

Here, in what used to be called Bandit Country, because the drumlins were full of gun-toting members of the Irish Republican Army, you come face-to-face with a legacy of The Troubles: the only place in Ireland where there are more gas stations than pubs.

Martin McAllister used to be one of those bandits. As a young IRA man, he came across a patrol of British Marines who opened fire on him. McAllister, barely old enough to shave, found himself on his back, bullet holes spread across his chest, and an enraged Marine about to finish him off with a shot to the head.

But a medic barked a warning at the Marine who was about to summarily execute McAllister, and told another Marine who was about to bash McAllister’s head in with his rifle to back off.

“I always wanted to meet that medic, to shake his hand, because he saved my life,’’ McAllister says, sitting at the kitchen table of his home, which sits in a bucolic part of County Monaghan in the Republic, surrounded by South Armagh in Northern Ireland.

The joke around South Armagh is that while Sinn Fein, the political party controlled by former IRA men like Gerry Adams and the late Martin McGuinness, campaigned against Brexit, the “Provos’’ privately voted for it, knowing it would boost the diesel business.

The Provos are the Provisional IRA, which are supposedly out of business, though no one around here believes that. Instead of selling unification, they’re selling fuel.

Five years ago, after McAllister publicly accused his erstwhile comrades in the IRA of monopolizing the fuel industry, some men dragged him from his car and beat him to within an inch of his life.

McAllister says the fact that Adams won’t even admit he was in the IRA is another example of the opportunistic, cold cynicism of the republican movement that led him to leave it.

But even someone as disillusioned with and opposed to the tactics and strategy of Sinn Fein as McAllister will admit the republican movement in particular and Northern Ireland in general lost an advocate for reconciliation when McGuinness died last year.

McGuinness, who, unlike Adams, openly acknowledged his role as an IRA leader, forged a genuine friendship with Ian Paisley, the fire and brimstone fundamentalist Protestant preacher who spent most of his life spitting out anti-Catholic bigotry. Then Paisley had a Damascus-like conversion and considered himself BFF with Martin McGuinness, and vice versa.

The unlikely friendship between McGuinness and Paisley was a metaphor for the new Ireland, where the animosities of the past were consigned to the dustbin of history. But they’re both dead now and so, according to McAllister and others, is the promise that the two biggest parties, Sinn Fein and the Democratic Unionists, would ditch their zero-sum sectarian mathematics: Catholics vote for Sinn Fein, Protestants vote for the DUP.

Ironically, McGuinness’s last act as a politician was to pull the plug on the power-sharing Northern Ireland Assembly of which he was co-leader. That assembly, in a bit of clever diplomacy, had forced nationalists and unionists to cooperate with each other politically if they wanted to control their own destinies. Alas, since McGuinness gave up on it, Northern Ireland has been ruled directly from London.

The political successors to that brave generation don’t exactly fill one with hope. The future is theirs and they seem determined to fumble it.

Arlene Foster, the DUP leader, is not exactly the leading candidate to make nice with the Shinners: Her father, a part-time policeman, was shot by the IRA. Foster appears in no rush to accommodate the people she long considered apologists for IRA violence.

Some hoped there would be a break in the stalemate and the polarized politics of the old Ireland when Adams stepped down in February as Sinn Fein’s party leader after 34 years. The so-called changing of the guard, from former IRA men to women who never took up arms, was seen as a potential game changer. But Michelle O’Neill,Sinn Fein’s leader at the suspended Northern Ireland Assembly, and Mary Lou McDonald, who took over as party leader, are not going to change Sinn Fein’s tack.

McDonald raised the ire of unionists when she gave a hearty endorsement to the IRA in February during her inaugural speech as party leader. She ended her speech by proclaiming “Up the rebels’’ and saying “Tiocfaidh ar la,’’ which in the Irish language means “Our day will come.’’

In a defense that strained all credulity, McDonald insisted she was referring to the “new Ireland’’ when using old IRA rallying calls.

But for all the focus on Ireland’s past, it is really what comes next that worries most people. Twenty years after a Good Friday when snow fell and scales fell away from the eyes of so many, the resumption of widespread violence isn’t what keeps people awake at night. It’s Brexit.

Some of the fears are mundane. Grainia Long, chief executive of the Irish Society for the Prevention of Cruelty to Children, lives in Belfast and insists that the plain people of Northern Ireland don’t sit around “talking about nationalism and a united Ireland’’ and how Brexit might affect that.

“They’re thinking about will they be able to drive down to Dublin for the rugby match,’’ she said.

Others are not.

“Most people are worried about health services,’’ she said, “not unification.’’

Long rejects suggestions that the imposition of direct rule after the assembly went into mothballs last year shows that nothing has changed since the bad old days. She said the failure of the local government controlled by unionists, and then London, to protect the rights of the Catholic minority was a hard-learned lesson.

“We have equality laws and laws that protect minorities that are among the best in Europe,’’ she said.

Chris McGimpsey, a Belfast city councilor, agrees.

McGimpsey’s political career is now threatened by illness: His Ulster Unionist Party got sick of him. They’re putting up another candidate in his constituency.

McGimpsey shrugs it off. Just as he shrugs off fears about Brexit, which he voted for.

“No one is going to put borders back up,’’ he said. “No one wants them.’’

I hope he’s right. But the truth is no one is sure of anything when it comes to Brexit.

The failed state of politics in the failed state of Northern Ireland leaves Ricky O’Rawe depressed. In his day, O’Rawe was a prominent IRA figure. He spent six years in prison for robbing a bank for the cause.

Now, O’Rawe avoids going to places like the Felons Club in West Belfast, where IRA veterans drain pints and tell war stories. He doesn’t go to IRA commemorations, and he doesn’t sit around fondly recalling the days when he and his comrades picked up guns to kill for Ireland. He thinks it was a waste. Of lives. Of aspirations. Of the normal human capacity to work out differences without resorting to violence.

But he doesn’t blame or begrudge the young people who picked up a gun because someone older and supposedly wiser convinced them to. In his middle age, he understands what has motivated men since the Middle Ages. That fear and insecurity make people do things they later regret. And he regrets what he did as an IRA man because it didn’t have to happen.

On Easter Sunday, Ricky O’Rawe and his wife, Bernadette, walked from their house to the republican plot at Milltown Cemetery, long after the official commemorations were over. He stood over the graves of his old comrades, young men who were once as idealistic as he was but never made it to middle age like he did.

Standing there, in the dead silence with the dead, Ricky O’Rawe told the men in graves that he still loved them and remembered them as if it was yesterday, when they were young and fearless and immortal and when, in what now seems like sheer madness, dying for Ireland was more important than living for it.

Kevin Cullen is a Globe columnist. He can be reached at cullen@globe.com.