LONDONDERRY, Northern Ireland — It began with a heavy-handed police raid on a suspected weapons cache, two minutes from Stevie Mallett’s youth club. Some residents responded with stones, gasoline bombs and, ultimately, bullets fired at the police and their vehicles. Then a young journalist lay dead.

For some in Northern Ireland, the riot here two weeks ago was an unexpected echo of a conflict that formally ended two decades ago. But for Mr. Mallett, it was to some extent predictable.

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The youth worker knows some of the young men involved personally — and has witnessed firsthand the deprivation that he believes partly led them to the latest incarnation of the Irish Republican Army.