The unrest emanates from the unionist community -- which believes in Northern Ireland's status as part of the UK
A week of rioting in Northern Ireland is the first evidence Brexit turbulence may be boiling into unrest in the British province, where post-EU rules are stoking fury among pro-UK sections.
The unrest emanates from the unionist community -- which believes in Northern Ireland's status as part of the United Kingdom -- and where some feel a new post-Brexit "protocol" is an existential threat.
Last week violence flared in the city of Londonderry, before spreading to the capital Belfast and outlying areas over Easter weekend and into Monday.
Small bands of masked males set cars alight and tossed petrol bombs and masonry at police, injuring 41 officers.
Police formed ranks with riot shields and armoured vehicles to retake the streets, arresting teens and young adults.
"There is no doubt that Brexit and the advent of the protocol has significantly damaged the balance of power," Ulster University politics professor Duncan Morrow told AFP.
Northern Ireland was the site of "The Troubles" -- a 30-year sectarian conflict that killed 3,500 people.
Unionist paramilitaries, British security forces and armed nationalists -- seeking to unite the territory with the Republic of Ireland -- waged battle until a landmark peace deal in 1998.
The accord let unionists and nationalists coexist by blurring the status of the region, dissolving border checks with fellow European Union member Ireland.
Britain's shock 2016 referendum decision to leave the bloc threatened to upset that arrangement, by requiring the return of the border checks.
Eventually a special "protocol" was agreed for Northern Ireland, shifting checks away from the land border to Northern Irish ports, on goods arriving from Britain.
It came into effect when a Brexit transition period ended at the start of 2021.
The new checks -- effectively keeping Northern Ireland in the EU's customs union and single market -- have already disrupted trade with mainland Britain.
But for many unionists the deeper issue is a sense of betrayal.
Regional justice minister Naomi Long, of the centrist Alliance Party, said British Prime Minister Boris Johnson's government "promised people unfettered access, which is not the case."
"They denied the existence of borders, even as those borders were being erected," she told the BBC on Wednesday.
Johnson's spokesman on Tuesday declined to comment on the factors driving the unrest, but offered sympathies for the injured officers and backed police appeals for calm.
Belfast Telegraph crime correspondent Allison Morris wrote that rioters have little interest in the complexities of UK-EU trade, "but they do know they're angry."
Some also believe that Brexit negotiators bowed to nationalists who tacitly threatened a bloody response to any resumption of checks on the Irish border.
Earlier this year, protocol checks were suspended at Belfast and Larne ports when graffiti appeared threatening staff.
Graffiti has also appeared in unionist areas depicting the gun crosshairs symbol and the phrase "all bets are off."