Joe Biden had spent a year in the hope that America could go back to normal. But last Thursday, the first anniversary of the deadly insurrection at the US Capitol, the president finally recognised the full scale of the current threat to American democracy.

“At this moment, we must decide,” Biden said in Statuary Hall, where rioters had swarmed a year earlier. “What kind of nation are we going to be? Are we going to be a nation that accepts political violence as a norm?”

It is a question that many inside America and beyond are now asking. In a deeply divided society, where even a national tragedy such as 6 January only pushed people further apart, there is fear that that day was the just the beginning of a wave of unrest, conflict and domestic terrorism.

A slew of recent opinion polls show a significant minority of Americans at ease with the idea of violence against the government. Even talk of a second American civil war has gone from fringe fantasy to media mainstream.

“Is a Civil War ahead?” was the blunt headline of a New Yorker magazine article this week. “Are We Really Facing a Second Civil War?” posed the headline of a column in Friday’s New York Times. Three retired US generals wrote a recent Washington Post column warning that another coup attempt “could lead to civil war”.

The mere fact that such notions are entering the public domain shows the once unthinkable has become thinkable, even though some would argue it remains firmly improbable.

The anxiety is fed by rancour in Washington, where Biden’s desire for bipartisanship has crashed into radicalized Republican opposition. The president’s remarks on Thursday – “I will allow no one to place a dagger at the throat of our democracy” – appeared to acknowledge that there can be no business as usual when one of America’s major parties has embraced authoritarianism.

Illustrating the point, almost no Republicans attended the commemorations as the party seeks to rewrite history, recasting the mob who tried to overturn Trump’s election defeat as martyrs fighting for democracy. Tucker Carlson, the most watched host on the conservative Fox News network, refused to play any clips of Biden’s speech, arguing that 6 January 2021 “barely rates as a footnote” historically because “really not a lot happened that day”.

With the cult of Trump more dominant in the Republican party than ever, and radical rightwing groups such as the Oath Keepers and Proud Boys on the march, some regard the threat to democracy as greater now than it was a year ago. Among those raising the alarm is Barbara Walter, a political scientist at the University of California, San Diego, and author of a new book, How Civil Wars Start: And How to Stop Them.

Walter previously served on the political instability taskforce, an advisory panel to the CIA, which had a model to predict political violence in countries all over the world – except the US itself. Yet with the rise of Trump’s racist demagoguery, Walter, who has studied civil wars for 30 years, recognized telltale signs on her own doorstep.

One was the emergence of a government that is neither fully democratic nor fully autocratic – an “anocracy”. The other is a landscape devolving into identity politics where parties no longer organise around ideology or specific policies but along racial, ethnic or religious lines.

Walter told the Observer: “By the 2020 elections, 90% of the Republican party was now white. On the taskforce, if we were to see that in another multiethnic, multi-religious country which is based on a two-party system, this is what we would call a super faction, and a super faction is particularly dangerous.”

Not even the gloomiest pessimist is predicting a rerun of the 1861-65 civil war with a blue army and red army fighting pitched battles. “It would look more like Northern Ireland and what Britain experienced, where it’s more of an insurgency,” Walter continued. “It would probably be more decentralized than Northern Ireland because we have such a large country and there are so many militias all around the country.”

“They would turn to unconventional tactics, in particular terrorism, maybe even a little bit of guerrilla warfare, where they would target federal buildings, synagogues, places with large crowds. The strategy would be one of intimidation and to scare the American public into believing that the federal government isn’t capable of taking care of them.”

A 2020 plot to kidnap Gretchen Whitmer, the Democratic governor of Michigan, could be a sign of things to come. Walter suggests that opposition figures, moderate Republicans and judges deemed unsympathetic might all become potential assassination targets.

“I could also imagine situations where militias, in conjunction with law enforcement in those areas, carve out little white ethnostates in areas where that’s possible because of the way power is divided here in the United States. It would certainly not look anything like the civil war that happened in the 1860s.”

Walter notes that most people tend to assume civil wars are started by the poor or oppressed. Not so. In America’s case, it is a backlash from a white majority destined to become a minority by around 2045, an eclipse symbolized by Barack Obama’s election in 2008.

The academic explained: “The groups that tend to start civil wars are the groups that were once dominant politically but are in decline. They’ve either lost political power or they’re losing political power and they truly believe that the country is theirs by right and they are justified in using force to regain control because the system no longer works for them.”

A year after the 6 January insurrection, the atmosphere on Capitol Hill remains toxic amid a breakdown of civility, trust and shared norms. Several Republican members of Congress received menacing messages, including a death threat, after voting for an otherwise bipartisan infrastructure bill that Trump opposed.

The two Republicans on the House of Representatives select committee investigating the 6 January attack, Liz Cheney and Adam Kinzinger, face calls to be banished from their party. Democrat Ilhan Omar of Minnesota, a Somali-born Muslim, has suffered Islamophobic abuse.

Yet Trump’s supporters argue that they are the ones fighting to save democracy. Last year Congressman Madison Cawthorn of North Carolina said: “If our election systems continue to be rigged and continue to be stolen, then it’s going to lead to one place and that’s bloodshed.”

Last month Congresswoman Marjorie Taylor Greene of Georgia, who has bemoaned the treatment of 6 January defendants jailed for their role in the attack, called for a “national divorce” between blue and red states. Democrat Ruben Gallego responded forcefully: “There is no ‘National Divorce’. Either you are for civil war or not. Just say it if you want a civil war and officially declare yourself a traitor.”

There is also the prospect of Trump running for president again in 2024. Republican-led states are imposing voter restriction laws calculated to favour the party while Trump loyalists are seeking to take charge of running elections. A disputed White House race could make for an incendiary cocktail.

James Hawdon, director of the Center for Peace Studies and Violence Prevention at Virginia Tech university, said: “I don’t like to be an alarmist, but the country has been moving more and more toward violence, not away from it. Another contested election may have grim consequences.”

Although most Americans have grown up taking its stable democracy for granted, this is also a society where violence is the norm, not the exception, from the genocide of Native Americans to slavery, from the civil war to four presidential assassinations, from gun violence that takes 40,000 lives a year to a military-industrial complex that has killed millions overseas.

Larry Jacobs, director of the Center for the Study of Politics and Governance at the University of Minnesota, said: “America is not unaccustomed to violence. It is a very violent society and what we’re talking about is violence being given an explicit political agenda. That’s a kind of terrifying new direction in America.”

While he does not currently foresee political violence becoming endemic, Jacobs agrees that any such unravelling would also be most likely to resemble Northern Ireland’s Troubles.

“We would see these episodic, scattered terrorist attacks,” he added. “The Northern Ireland model is the one that frankly most fear because it doesn’t take a huge number of people to do this and right now there are highly motivated, well-armed groups. The question is, has the FBI infiltrated them sufficiently to be able to knock them out before they they’ve launch a campaign of terror?”

“Of course, it doesn’t help in America that guns are prevalent. Anyone can get a gun and you have ready access to explosives. All of this is kindling for the precarious position we now find ourselves in.”

Nothing, though, is inevitable.

Biden also used his speech to praise the 2020 election as the greatest demonstration of democracy in US history with a record 150 million-plus people voting despite a pandemic. Trump’s bogus challenges to the result were thrown out by what remains a robust court system and scrutinised by what remains a vibrant civil society and media.

In a reality check, Josh Kertzer, a political scientist at Harvard University, tweeted: “I know a lot of civil war scholars, and … very few of them think the United States is on the precipice of a civil war.”

And yet the assumption that “it can’t happen here,” is as old as politics itself. Walter has interviewed many survivors about the lead-up to civil wars. “What everybody said, whether they were in Baghdad or Sarajevo or Kiev, was we didn’t see it coming,” she recalled. “In fact, we weren’t willing to accept that anything was wrong until we heard machine gun fire in the hillside. And by that time, it was too late.”