LONDON — A bus hijacked, pelted with stones, then set on fire. Masked youths rioting, hurling missiles and homemade bombs. A press photographer attacked on the streets.
For almost a week, scenes of violence familiar from Northern Ireland’s brutal past have returned in a stark warning of the fragility of a peace process, crafted more than two decades ago, that is under growing political and sectarian strain.
Amid a contested fallout from Brexit, politicians have pointed to different causes for an explosion of anger from parts of the Protestant, so-called Unionist or Loyalist, community that is determined to keep its link to the rest of the United Kingdom.
But analysts agree that six consecutive nights of violence, during which 55 police officers have been injured and 10 arrests made, mark a worrisome trend.
“I think it’s very serious. It’s easy to see how things can escalate and hard to see how things can calm down,” said Katy Hayward, a professor of political sociology at Queen’s University, Belfast.
In the febrile aftermath of Brexit, she added, Unionists “feel betrayed by the British government and feel that Northern Ireland’s place in the union is very much under pressure as a result, so that sense of insecurity definitely raises the stakes.”
Jonathan Caine, a Conservative Party member of the House of Lords and former adviser to six Northern Ireland secretaries, said the violence reflected dangerous tensions.
“By historic standards it is not out of control, but it could be and the reason is not just the reaction to Brexit,” he said.
“There are deep-seated anxieties within the Unionist community and a perception that they have been left behind, that everything is geared not to them but to the Republicans,” he added, referring to parts of the Roman Catholic population who favor a united Ireland.
With rioting by some as young as 13, the violence has shocked politicians, prompting condemnation from Prime Minister Boris Johnson of Britain and Northern Ireland’s power-sharing executive, which on Thursday called for calm to be restored. On Thursday, bus drivers parked outside City Hall to protest an incident in which one of their colleagues had his vehicle hijacked and burned.
Adding to the concerns, the latest violence took place in sensitive parts of Belfast on the border between areas populated by mainly Protestant communities and those where Roman Catholics mostly live, raising the risks of a violent response.
Despite the 1998 Good Friday Agreement that largely ended decades of bloodshed known as the Troubles, neither sectarian violence, nor the paramilitary groups behind it, have ever fully disappeared from Northern Ireland.
Some people believe that shadowy groups are exploiting sectarian anxieties and frustrations with Covid-19 restrictions to cause problems for police officers who have been clamping down on the groups’ criminal activities.
Though tensions have risen in recent weeks, it was an incident dating back many months that was the catalyst for the most recent violence, which saw rioters burning tires and garbage in the streets.
In June 2020, despite Covid-19 rules banning large gatherings, the police allowed a funeral to go ahead following the death of Bobby Storey, who was considered the head of intelligence of the Irish Republican Army, an armed group dedicated to a united Ireland that waged a violent campaign against British forces during the so-called Troubles.
Among around 2,000 people who attended his funeral were senior members of Sinn Fein, a party that mainly represents Roman Catholic voters. The party was once seen as the political wing of the I.R.A. but now plays a prominent part of the democratic power-sharing system in Belfast.
A decision last week not to prosecute mourners for breaking Covid regulations infuriated Unionists, sparking protests and prompting Northern Ireland’s first minister, Arlene Foster, to demand the resignation of the police chief, Simon Byrne, over his handling of the funeral.
Mr. Caine said that in Northern Ireland policing decisions are particularly difficult given the risk of stoking disorder, and that the security forces can be placed in an impossible position. Nonetheless the lack of prosecutions “played into the feeling among some Unionists that it is one rule for Sinn Fein and another for the rest of us,” he said.
Ever since the 1998 peace deal there has been dissatisfaction among some Unionists “and a perception that it was a victory for the Republicans, that they have all the benefits and the Loyalists haven’t got anything,” he added.
But tensions had also been building since Britain completed the final stages of Brexit on Jan. 1. That ended a system under which companies in Northern Ireland shared the same trade rules as those of Ireland, which remains part of the European Union.
During the interminable Brexit negotiations, much energy was devoted to preventing the need for checks on goods at Northern Ireland’s highly sensitive land border with Ireland.
Under an agreement in a protocol struck by Mr. Johnson, Northern Ireland was given a special economic status that leaves it straddling the United Kingdom and the European Union trade systems.
However, it also imposes some new checks, particularly on goods flowing from mainland Britain to Northern Ireland — something that is anathema to Unionists who want equal treatment with the rest of the United Kingdom.
Despite the deal he signed up to, Mr. Johnson promised that there would be no new “border in the Irish Sea,” and, glossing over the looming difficulties, his government did little to prepare opinion in Northern Ireland for the changes.
Yet on Jan. 1, when the post-Brexit trade rules came into force, businesses faced new paperwork and some British companies stopped moving goods to Northern Ireland, causing some shortages on supermarket shelves. Amid rising tensions, checks on goods were halted temporarily after threats were made against customs staff.
“Unionists were told by Boris Johnson there wouldn’t be a border in the Irish Sea. Even on Jan. 1 they were told that we will never see the integrity of the United Kingdom’s single market undermined, so they feel betrayed by the protocol,” said Professor Hayward.
For Northern Ireland’s biggest political force, the Democratic Unionist Party, led by Ms. Foster, the situation is particularly delicate. It campaigned for Brexit and opposed a softer version that was proposed by the former British prime minister, Theresa May, only to end up with one that brings exactly what it did not want: a more tangible and visible separation between Northern Ireland and the rest of the United Kingdom.
However, the European Union contributed to the crisis too when it briefly announced in January plans to effectively suspend the protocol by triggering an emergency mechanism in a dispute over vaccine supplies. Though the British government had also threatened to break the treaty over a separate issue — and the European Union reversed its decision within hours — that united Unionists in anger.
“Those few hours on Jan. 29 changed everything,” said Professor Hayward, who added that the decision from Brussels encapsulated Unionist suspicions about the protocol and shifted senior politicians away from grudging acceptance of it to outright opposition.
With Unionist support for the protocol disappearing, faith in the police in question, and friction over Brexit between the British and Irish governments, calming the violence could prove hard.
“In the past these things have been mitigated by very careful, well-supported actions by community workers on the ground, bolstered by the political environment and rhetoric and demonstrations of the success of peace at the very highest levels — including the British-Irish relationship,” said Professor Hayward.
“You look around now,” she added, “and think: All those things are really under pressure.”