As a former overseas operative who has struggled both on the side of insurgents and against them, the past few days have brought a jarring realization: We may be witnessing the dawn of a sustained wave of violent insurgency within our own country, perpetrated by our own countrymen. Three weeks ago, it would have been unthinkable that the United States might be a candidate for a comprehensive counterinsurgency program. But that is where we are.

Overrepresented among the ranks of angry but ordinary citizens who stormed the Capitol on Jan. 6 were others, hardly ordinary, committed to violent extremism: the Proud Boys, the Three Percenters, the Oath Keepers, “Christian” national chauvinists, white supremacists and QAnon fantasists, among others. Some of these groups may have planned their incursion in advance, but they could not have breached the Capitol if not for the wave of populist anger that swept them forward and over the barricades.

Given impetus and, they believed, political cover by former President Donald Trump, the capering idiots who filmed themselves in the Capitol seemed to think they were untouchable. They may be easy to identify and arrest now, but there are others — well armed, dangerous and now forewarned — who had a glimpse of what may be possible in the political environment Mr. Trump created.

There has long existed in this country a large, religiously conservative segment of the population, disproportionately (though not entirely) rural and culturally marginalized, that believes with some reason it is being eclipsed by a politically and culturally ascendant urban coalition of immigrants, minorities and the college-educated secular elites of tech and mainstream media. That coalition, in their eyes, abridges their religious freedoms, disparages and ‘cancels’ their most cherished beliefs, seeks to impose ‘socialism’ and is ultimately prepared to seize their guns.

This, in very general terms, is the core segment of the nation that has been unified, championed and politically energized by Donald Trump.

Bridging the urban-rural cultural and political gap with facts, tolerance and empathetic sincerity is a vital national project, but one which has become effectively impossible. The sincere belief, reportedly held by a majority of Republicans, that the Democrats stole the recent national election through massive fraud has taken the longstanding fears and resentments of a large section of our fellow citizens to a new and qualitatively different level.

In context, their fury is understandable. If I believed as they do, I would be marching with them. The Big Lie perpetrated by Mr. Trump and his allies in the political class and among large elements of the right-wing media, preposterous as it may be, will have incalculable implications not just for long-term political comity in this country, but also for national security.

The violent demonstrations feared for Inauguration Week, in the face of extraordinary security precautions, didn’t materialize. Relatively few of our citizens would embark on a program of sustained violence in any case. But if popular anger has crested, left in its wake is a bitter, simmering restiveness, one that will provide a nurturing environment for the worst among us — the extremists who seek a social apocalypse. Their numbers may be relatively small, but even a small slice of a nation of over three hundred million is substantial. Without a program of effective national action, they and their new adherents are capable of producing endemic political violence of a sort not seen in this country since Reconstruction.

The challenge facing us now is one of counterinsurgency. Though one may recoil at the thought, it provides the most useful template for action, which must consist of three elements.

First, the easiest and most straightforward, is criminal justice. We should continue to track known extremists, and investigate and bring to account those who commit crimes. We have the expertise and the infrastructure to do so, and to do it while preserving civil liberties. We need no new statutes, nor should we import terrorist designations that should apply only to foreign groups beyond the reach of domestic law.

But the first element will not succeed without a second, which is even more important but far more difficult: We must isolate and alienate the committed insurgents from the population. Just as Al Qaeda in Iraq depended on a much larger community of disaffected Sunnis for tacit support and recruitment, we face the prospect of there being a mass of citizens — sullen, angry and nursing their grudges — among whom the truly violent minority will be able to live undetectably, attracting new adherents to their cause.

The fantasy that the presidency was stolen from Mr. Trump, which has gripped so much of the country, will not easily be broken. The nation is in an epistemological crisis. When “facts” become untethered from objective reality, they become excuses to justify what one wants to believe. Yes, the problem is far worse on the right than on the left, but the problem is a general one.

We must establish, undeniably, what actually happened in the election. That requires neither new laws nor a thought police: It’s not something for the government, but for all of the nation. We must all earnestly engage in an effort to listen to others’ ideas, no matter how daft they may seem; to understand where such ideas come from, no matter how hateful the source; to meet assertion with reason and evidence, not counterassertion. And where our evidence is lacking, we must patiently seek it out.

Neighbor must speak with neighbor across the divide, rather than merely shunning alien views. Media figures must concede inconvenient facts, rather than tarring the other side with an emotionally satisfying broad brush. This is far from saying that all thoughts and ideas have equal validity: They do not. But truth is unavailing if not presented with clear underlying fact, and if not conveyed with respect. Success in restoring evidence-based truth as the language of public discourse is by no means assured, but lack of effort will doom us to failure.

To be sure, the nation’s fundamental and legitimate political divisions will remain. But while not all differences can be bridged, they can be tolerated. For their part, Democrats would do well to avoid the more extravagant aspects of their agenda, which might confirm the worst fears of the rural heartland. By bringing people together, we can isolate the extremists.

The final element of the plan concerns insurgency leadership. Mr. Trump’s transition from mere subversion of the constitutional order to open incitement of mass violence exposes what he has long represented to the most radical fringe of his supporters: a charismatic symbol. By shamelessly espousing the politics of white grievance and convincing so many that he actually won re-election, Mr. Trump has created the conditions necessary for the extremists’ success. They know better than to take his recent, ritualistic admonitions against violence at face value, and so should we. He will continue to be their champion, and his self-serving lies will be their most potent enabler.

As the Senate prepares to sit in judgment on Mr. Trump, we should be wary of the excuses put forward by his defenders — that his conviction will only divide the country further, that we should simply move on. No: It is far too late for appeasement. Those of us versed in counterinsurgency know that in violent extremism nothing succeeds like success, and that the opposite is also true.

I watched as enraged crowds in the streets of Algiers, as in most Arab capitals, melted away when Saddam Hussein was ignominiously defeated in the Persian Gulf war. Mass demonstrations in Pakistan in support of Osama bin Laden fell into dull quiescence when he was driven into hiding after Sept. 11. To blunt the extremists, Mr. Trump’s veneer of invincibility must similarly be crushed.

Defeating him politically was the first step. Given the continuing threat he poses, convicting him in the Senate and barring him from future elective office is not only a just punishment for his crimes but also a national security imperative. He will, and must, retain his First Amendment rights. But the public shunning and permanent diminishment of Mr. Trump is a necessary requirement of future peace.

The political and social divisions in our country will take time and application, from both sides, to heal. In the meantime, we minimize the threat at our peril.

Robert Grenier is a former intelligence officer who was director of the C.I.A.’s Counterterrorism Center from 2004 to 2006.

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