“Those few hours on Jan. 29 changed everything,” said Professor Hayward, who added that the decision from Brussels encapsulated Unionist suspicions about the protocol and shifted senior politicians away from grudging acceptance of it to outright opposition.

With Unionist support for the protocol disappearing, faith in the police in question, and friction over Brexit between the British and Irish governments, calming the violence could prove hard.

“In the past these things have been mitigated by very careful, well-supported, actions by community workers on the ground, bolstered by the political environment and rhetoric and demonstrations of the success of peace at the very highest levels — including the British-Irish relationship,” said Professor Hayward.

“You look around now,” she added, “and think: all those things are really under pressure.”

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