The loyalist marching season kicks off in Northern Ireland at a time of growing tensions, driven by discontent over Brexit, that is also causing divisions within the largely Protestant unionist community.

DERRY, Northern Ireland — The curbs are painted the blue, red and white of Britain’s Union Jack in the Fountain housing estate, the only Protestant enclave in this part of Derry, Northern Ireland. The ashes of a bonfire fueled with the tricolor flag of neighboring Ireland lay in a central square.

Along these narrow streets, bands from the Protestant community marched on Monday to mark July 12, a commemoration of a centuries old military victory of a Protestant king over a Catholic one.

Such marches are a longstanding annual event in Northern Ireland, but the tensions growing over changes that Brexit has wrought in the region are casting the parades in a new light. There has been sporadic violence in recent months, and fears that the tense climate could threaten the landmark 1998 Good Friday Agreement that ended decades of sectarian strife and halted a 30-year conflict in Northern Ireland.

Across the region last weekend, bonfires blazed ahead of the parades, as towers of teetering pallets were set alight, casting a flickering orange glow on the faces of onlookers who gathered for street parties. This year, there are two additional dynamics at play — the centenary of the partition of Ireland that established Northern Ireland, and ongoing discontent with the post-Brexit trade arrangements for the region, known as the Northern Ireland protocol, that have heightened long dormant tensions.

The worries are centered within the mostly Protestant Unionist community, where tensions have grown over its relationship with the rest of Britain.

The protocol, a deal reached between the British government and Europe to avoid resurrecting a hard border between Northern Ireland and the Republic of Ireland, has come to embody broader discontent from unionists over neglect of the region by Westminster.

Many unionists feel alarmed or are resentful about the British government’s agreement with Europe, said Katy Hayward, professor of political sociology at Queen’s College in Belfast.

And Irish nationalists are upset that Northern Ireland is being removed from the European Union against the wishes of the majority who voted to remain in the bloc, she said.

While the Good Friday Agreement halted the violence, known as the Troubles, it failed to address the underlying sectarian roots and created a “fragile balance,” Ms. Hayward said, which depended on cooperation between Britain and Ireland, north and south, and unionists and nationalists.

“Across all three strands of the Good Friday agreement, that balance, the thing that has kept it in place has been taken away,” she said. “So everybody’s feeling that particular degree of insecurity.”

Members of the Orange Order, a religious and political Protestant fraternal order, march in the city — which is also called Londonderry by unionists who want the region to remain part of the United Kingdom — and lead the festivities marking William of Orange’s military victory over the Catholic King James II in 1690.

Many Catholic nationalists see the traditions associated with such celebrations, like the Orange Order marches and bonfires, on which the Republic of Ireland’s tricolor flag are often burned, as a provocation. Caoimhe Archibald, a local Sinn Fein politician — an Irish Republican party — shared an image of one of the bonfires painted in the tricolor on Twitter with the message: “This isn’t an expression of culture, it’s an expression of hate.”

But many Protestants maintain it is a vital celebration of identity and heritage.

“It’s a culture I’ve been brought up on, it’s a culture I’m proud of,” said William Jackson, 59, a day earlier as he played outside with his grandchildren in the Fountain estate ahead of the annual celebration. The neighborhood is encircled by a high metal fence. British flags are duct taped to lamp posts wrapped in barbed wire.

Born and raised in Derry, Mr. Jackson remembers well the old conflict — between Catholic nationalists, who more closely identify with the Republic of Ireland, and predominantly Protestant loyalists and unionists, who see themselves as British — and worries it would take little to set off renewed violence.

“That could all start again tomorrow,” said Mr. Jackson. “It doesn’t take much to light a fuse, in my opinion, that is just waiting to happen. Because sooner or later the Protestant community who have been let down on all occasions are going to stand up and say right, we’ve had enough of this.”

While the marches passed without incident around the region on Monday, some Derry residents were unimpressed that they had been allowed to continue amid the pandemic. A group of Catholic women watched with folded arms from a doorway as the parade passed by, and said they believed the marches only make matters worse.

The marches normally feature dozens of bands and draw thousands of spectators. This year, they were divided into a series of smaller neighborhood marches because of the pandemic. Dozens of bonfires were also lit over the weekend in estates in loyalist areas in an atmosphere that was simultaneously festive and fraught.

“It’s almost been the perfect storm,” said Brian Dougherty, a community worker in Derry, who noted he has seen a shift in communal relations in the city after the 2016 Brexit referendum. “We were talking all along quite comfortably up until then and then this thing happened.”

Mr. Dougherty said that the community had made great strides toward long-term peace building, but that the general anti-British sentiment caused by Brexit has also created a “hostile atmosphere” among unionists, pushing many to reaffirm their identities as part of the United Kingdom.

“What we found here,” he said, “is all of a sudden curb stones were starting to be painted red, white and blue, flags were getting flown again, the bonfires were getting higher.”

Despite the internal divisions, the celebrations still play a key role in reinforcing loyalist identity. Mr. Dougherty noted that the bands in particular have created a positive space for young people.

Most working class loyalist estates have a marching band that tends to attract some of the most marginalized, disenfranchised young people, he said, especially young men who may otherwise turn to paramilitaries. Two decades ago, the bands themselves were sometimes magnets for loyalist paramilitaries, “but that mentality has changed,” Mr. Dougherty said.

“It’s about time we had a more honest reflection what parading means and what the positivity can bring, particularly to disenfranchised young people,” he said, adding that there was broad community work being done, including relationships being built with Catholics. “It’s a really important message, that we get away from the binary politics of green and orange. There’s a lot of nuances in between.”

Julie Porter, 27, who marched in Derry on Monday, has played flute in the band for 12 years and sees it as a way to celebrate traditions and build self-confidence.

For those who grow up in working class areas, she said, “there is not a lot for you to do,” and the bands offered an alternative, particularly for young men.

“And actually a band gives a different form of leadership and can take them off that path and onto a better one,” she said.

In the port city of Larne, two towering bonfires made of wooden pallets were stacked in looming tiers and drew large crowds on Sunday night in the neighboring Craigyhill and Antiville estates before they were set alight just after midnight.

Paramilitary groups have increasingly played a role in building the pyres at these particular bonfires that once had been mostly created by the community, locals said. Flags celebrating local militias flew at the Craigyhill estate on Sunday night and a brigade boss was pictured posing atop the pile.

But many say the bonfires are merely a celebration, and a long overdue reunion with neighbors and friends after a year of pandemic restrictions.

Families shared drinks in front yards under Union Jack banners as two children ran by with flags tied around the shoulders. A little girl turned cartwheels in the glow of the fire. The crowd cheered as a pile of pallets teetered steeply and collapsed into a pile of flames, throwing ash skyward. But the ongoing controversy about the Northern Ireland protocol has also exposed deep divisions within unionist communities.

“I wanted Brexit, but we didn’t vote for the Northern Ireland protocol,” said Ruth Nelson, 41, who was visiting her sister in the Antiville estate in Larne for the bonfire. “England screwed us again.”

She said she feels forgotten, by London and by local unionist politicians. Unionism in Northern Ireland is reaching a crisis point, experts and members of the community say, as Brexit widens divisions within the movement.

Many unionists feel the British government betrayed and misled them, Professor Hayward said.

“They depend on the British government at the same time as not trusting them,” she said. “The lessons of history suggests that they’re wise to be cautious, and I think it’s fair to suggest they will be let down again.”