© Peter Muhly/Agence France-Presse — Getty ImagePolice officers in Belfast, Northern Ireland, remained with their armored vehicles as a car burned after violence between unionists and loyalists on Jan. 12.
For more than six weeks, it has been a dismal case of back-to-the-future, a crudely sectarian upheaval that has defied all attempts at peacemaking.

The scenes recall the sectarian bitterness that infused the 30 years of virtual civil war known as the Troubles: night after night of street protests marshaled by balaclava-wearing militants, who have updated their tactics by using social media to rally mobs; death threats to prominent politicians, some of whom have fled their homes and hidden under police guard; firebombs, flagstones and rocks hurled at churches, police cars and lawmakers' offices; protesters joined by rock-throwing boys of 8 and 9; neighborhoods sealed off for hours by the police or protesters' barricades.

Many had hoped that the old hatreds between Northern Ireland's two main groups - the mainly Protestant, pro-British unionists, and the mainly Roman Catholic republicans, with their commitment to a united Ireland - would recede permanently under the auspices of the Good Friday agreement. That accord was reached 15 years ago as a blueprint for the power-sharing government that now rules the province.

But the fragility of those hopes has been powerfully demonstrated by more than 40 days and nights of violence that were triggered by a decision to cut back on the flying of the Union Jack, Britain's red, white and blue national flag, over the grandly pillared, neo-Classical City Council building in central Belfast.

By the latest count, more than 100 police officers have been injured, along with dozens of protesters and bystanders. At times, the violence has expanded to other cities, including Londonderry. Business has slumped. Police commanders, their forces overwhelmed, have assigned dozens of officers to scan hundreds of hours of closed-circuit video, looking for ringleaders.

The crisis began modestly enough. The Belfast council, its pro-British members outvoted by a coalition of republicans and a small liberal bloc, decided in early December to limit the flag flying to 18 days a year, as specified by London for all of Britain. Through the decades when the council was dominated by Protestant unionists, committed to links with Britain, the flag flew from the pinnacle of the building every day of the year.

Incongruously, perhaps, most of those 18 days do not represent landmarks in Britain's history - Nelson's victory at Trafalgar, say, or Germany's surrender in the Second World War - but the birthdays of Queen Elizabeth II and her family members, including the former Kate Middleton, now the Duchess of Cambridge, on whose 31st birthday, Jan. 9, the Belfast flag fluttered for the first time since it came down in early December. Under Britain's strict rules about flying the national standard on public and private buildings, not even the Parliament buildings in London fly it on any but government-designated days. But the hauling down of the Belfast flag provoked a furious reaction, the most protracted period of unrest in many years in Northern Ireland.

Among pro-British loyalists, the episode was seen as part of the step-by-step erosion of the British presence, a stripping of what many of them call their identity. Other examples they invoke have also been symbolic, including moves to delete the word Ulster - an ancient designation for the northern Irish provinces commonly used by Protestants but mostly shunned by republicans - from the formal names of the province's police force and its military reservists, and to remove the British crown emblem from the cap and shoulder badges of prison guards and other public officials.

But many of the province's political commentators see the flag dispute as a token of something more profound and ultimately more threatening to the hopes for a permanent peace here.

They say the council's decision on the flag, made possible by the fact that nationalists now hold 24 seats on the council, compared with 21 for the unionists, reflects the rapid growth of the Catholic population in the years since the Good Friday agreement, unsettling the long-held assumption among unionists that Protestants would constitute a permanent majority in the province.

The most recent census results, released last year, showed that 48 percent listed themselves as Protestant or brought up Protestant, down 5 percentage points from the 2001 census, while 45 percent of the population listed themselves as Catholic or brought up Catholic, a 1 percentage point rise. In Belfast, many say, Catholics are already a majority or nearly so and could form a majority across the province within a decade.

Since the Good Friday agreement specified that the province would remain part of Britain as long as a majority of the province's people and of the population of the whole of Ireland wished it to be, the reasoning goes, Protestants who are resolved never to accept a united Ireland could be right in seeing the flag dispute as a harbinger of their worst fears.

Patricia MacBride, a Catholic whose father was killed by loyalist gunmen and who has been a leader in reconciliation efforts under the Good Friday agreement, said she had always feared that the peace process might founder when Protestants realized that the population numbers were moving against them. With the sense that power was shifting away from them, she said, the sense of betrayal among Protestants had intensified.

"There was always going to be something that triggered this upheaval," she said. "Increasingly, they feel abandoned by the state whose agents they have been for so long."

Strong backing for that view was evident on a recent morning in Belfast on Shankill Road, a depressingly run-down loyalist stronghold notorious as a center of sectarian ambushes, bombings and shootings during the Troubles. There the Union Jack was everywhere, atop buildings, in shop windows, in tattoos and in the hands of small children out shopping with their parents. In bars, cafes and shops, "the humiliation" of the flag issue was the center of conversation. In the winter chill, groups of men, mostly unemployed, reinforced one another's indignation.

On a stretch of the road punctuated with memorials to Protestants killed in the Troubles and to Ulstermen who died in World War I, Paul Shaw, 33, owner of the Shankill Band Shop, boasted of doing a roaring trade during the upheaval, selling thousands of flags and other loyalist memorabilia, including DVDs of patriotic songs sung by Ulstermen on the battlefields of the Somme.

"It's our flag, our identity; it's been flown above City Hall every day since 1906, and it's being stripped from us," he said. With nods from others clustered around him, he compared the flag battle to the fighting on the Somme. "If we lose this one, we'll have a united Ireland in 5 or 10 years, and we won't accept it," he said. "We'll die to defend the flag. If we have to, we'll go back to the graveyards and the jails."

With no end in sight, leaders of the power-sharing government have voiced anxiety that the protests, by whipping up antagonism among Protestants, could threaten the peace process. But the top two officials in the power-sharing administration - Peter Robinson, the unionist first minister, and Martin McGuinness, the republican first deputy minister, who is Mr. Robinson's effective coequal - vowed in the province's assembly on Monday that their commitment to the peace agreement would not be shaken. Mr. Robinson was unsparing in his rebuke to the protesters. "You do not respect the union flag if you are using it as a weapon," he said, adding that the protests were "a cynical cover for the real political agenda, which is to destroy the political process."

Mr. McGuinness, a former chief of staff of the Irish Republican Army, seemed eager not to draw sectarian comfort from the turmoil in the unionists' political base.

"I do not believe for a moment that they speak for the vast majority of unionists," he said of the flag protesters, dismissing their efforts as a crude challenge to the power-sharing arrangements "from people who do not have a mandate and speak for nobody but themselves."