LONDON (AP) — Young people have hurled bricks, fireworks and gasoline bombs at police and set hijacked cars and a bus on fire during a week of violence on the streets of Northern Ireland.

The chaotic scenes have stirred memories of decades of Catholic-Protestant conflict, known as “The Troubles.” A 1998 peace deal ended large-scale violence but did not resolve Northern Ireland’s deep-rooted tensions.

The situation deteriorated into a conflict between Irish republican militants who wanted to unite with the south, loyalist paramilitaries who sought to keep Northern Ireland British, and U.K.

Most were in Northern Ireland, though the Irish Republican Army also set off bombs in London and other British cities?

that has a border with an EU nation — Ireland — it was the trickiest issue to resolve after Britain voted narrowly in 2016 to leave the 27-nation bloc.

An open Irish border, over which people and goods flow freely, underpins the peace process, allowing people in Northern Ireland to feel at home in both Ireland and the U.K.

The alternative was to put it, metaphorically, in the Irish Sea — between Northern Ireland and the rest of the U.K.

That arrangement has alarmed British unionists, who say it weakens Northern Ireland’s place in the United Kingdom and could bolster calls for Irish reunification?

The violence has been largely in Protestant areas in and around Belfast and Northern Ireland’s second city, Londonderry, although the disturbances have spread to Catholic neighborhoods.

Border staff were temporarily withdrawn from Northern Ireland ports in February after threatening graffiti appeared to target port workers.

Some in Northern Ireland’s British loyalist community feel as if their identity is under threat.