Adding to the world’s sectarian flash points, the British territory of Northern Ireland has roared back into the news, its relative calm punctured by violent rioting among groups that had made peace 23 years ago.

The reasons for the breakdown are intertwined with Britain’s exit from the European Union and the stresses of the Covid-19 pandemic. But they have demonstrated the combustible potency of the old feuds between a largely Catholic side that wants the territory to be part of Ireland, and a mostly Protestant side that wants to remain part of Britain.

For more than a week, protests have descended into mayhem in the streets of Belfast, the capital, and some other parts of Northern Ireland, leaving scores of police officers wounded. Rioters as young as 13 have thrown gasoline bombs at the police and set buses afire. Prime Minister Boris Johnson of Britain and his Irish counterpart, Micheal Martin, have both expressed deep concern.

“Boris Johnson is wrestling with a problem that is too close to home for comfort: the worst violence on the streets of Northern Ireland for many years,” Mujtaba Rahman, managing director Europe for the Eurasia Group, a political risk consultancy, said in an email to clients. The underlying causes, Mr. Rahman said, “were unlikely to be resolved quickly.”

Here is a look at Northern Ireland and the issues behind its violent turn.

Northern Ireland is a 5,400-square-mile area of roughly two million people under British sovereignty in the northeast part of the isle of Ireland, bordered on the south and west by the Republic of Ireland and on the east by the Irish Sea, which separates it from the rest of Britain.

Ireland became self-governing almost 100 years ago after centuries of British rule. But the treaty that established self-rule for most of the island, after several years of fierce struggle in the wake of World War I, also contained an opt out for the area with the largest concentration of Protestants, whose leaders strongly opposed the prospect of becoming part of a Catholic-majority state. This northern area remained part of Britain, with a police force and a local government dominated for decades by Protestants.

The division of Ireland became the source of one of the 20th century’s most violent and enduring sectarian conflicts, pitting Catholics and groups opposed to British rule, including the paramilitary Irish Republican Army, against Protestants and pro-British forces including loyalist militant groups. Belfast, a onetime shipbuilding epicenter and birthplace of the Titanic, became one of the “four Bs” — joining Beirut, Baghdad and Bosnia in the pantheon of the world’s most perilous places. Roughly 3,600 people died in decades of strife in Northern Ireland known as “The Troubles.”

An accord known as the Belfast Agreement, also called the Good Friday Agreement or simply the agreement, was reached on April 10, 1998, by the British government, the Irish government and Northern Ireland political parties. It created a governing assembly for the territory designed to ensure power-sharing between Protestants and Catholics, and bodies to ease cooperation between Northern Ireland and Ireland. It committed former adversaries to disarm and settle their disputes peacefully. It also permitted residents of Northern Ireland to obtain Irish citizenship or dual Irish-British citizenship.

Years of relative peace followed. Once considered a no-go area for tourists, Northern Ireland became a draw. Its attraction was further enhanced by the creators of “Game of Thrones,” the HBO series, who used its stunning and diverse landscapes as their stage. The show’s April 2011 debut put “the north of Ireland on the map,” said The Derry Journal, a newspaper in Northern Ireland’s second-largest city.

Britain’s departure from the European Union, known as Brexit, disturbed the political balance in Northern Ireland, threatening the underpinnings of the Good Friday Agreement.

Ireland remains a European Union member country, and Brexit raised the prospect of new checks at its previously unrestricted land border with Northern Ireland, impeding the free flow of people and goods and angering those who would like to see the island unified.

But workarounds to keep that border open have created new problems in commerce between Northern Ireland and the rest of Britain, disrupting supplies to the territory’s stores and upsetting those in Northern Ireland who see themselves as British. Resentment in pro-British Protestant areas has swelled and contributed to the most recent outbreaks of violence, raising fears of retaliation from Catholic communities.

A further source of tension was a recent police decision not to prosecute crowds of mourners who gathered at a funeral last June for Bobby Storey, an Irish Republican Army commander, despite a ban on mass gatherings because of the pandemic. Among the mourners were leaders of Sein Fein, a political party with links to the I.R.A. that has become the leading party among Northern Ireland’s Catholics.

While there are no expectations that the violence will escalate to levels seen during the years of The Troubles, when British forces were deployed to Northern Ireland, leaders on all sides fear the onset of a cycle of revenge attacks.

Northern Ireland’s predicament has now become an especially delicate issue for Mr. Johnson’s government. He does not want to lose support from Protestants in Northern Ireland who say they feel betrayed and disenfranchised. And any deepening of divisions between Northern Ireland and Ireland could galvanize support for Irish unification, which some polls suggest has already risen since Brexit.

For now, political leaders on all sides are emphasizing the need to honor the 1998 Belfast Agreement, reminding Northern Ireland’s young people how it transformed their lives. Mr. Martin, Ireland’s prime minister, put it this way in remarks on Saturday, the agreement’s anniversary: “We owe it to the agreement generation and, indeed, future generations not to spiral back to that dark place of sectarian murders and political discord.”

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