Terrorists murdered three people in Northern Ireland last week. A decade ago that news would have been commonplace. Today, however, it is a stunning development for a people who have grown accustomed to peace and reconciliation. The remaining elements of a terror movement are trying to fan the flames of dissent and unravel the peace accord that has transformed Northern Ireland: They cannot be allowed to succeed.

Ten years ago, Catholics and Protestants who had fought a vicious bloody battle for control of Northern Ireland signed a peace agreement. The Good Friday accord ended “the Troubles” — three decades of fighting that claimed more than 3,500 lives, more than half of them civilians. The peace process was uneven, but it yielded the 2005 decision of the Irish Republican Army to disarm and the formation in 2007 of a joint Catholic-Protestant government. Today, a Protestant first minister works side by side with a former Catholic IRA commander who is deputy first minister.

While the overwhelming majority of Northern Ireland’s citizens welcomed the peace, a small hard core of dissidents has not. They have engaged in small acts of violence: According to the police, dissidents have launched more than 20 gun, rocket and bomb attacks since November 2007, wounding several police officers but killing no one.

That changed last week. As four British soldiers went to the main gate of their barracks to pick up a final pizza before they departed for duty in Afghanistan, gunmen opened fire, killing two of the soldiers and wounding others, including the pizza deliverymen. The gunmen dismissed the shooting of civilians as proper punishment for collaborators. A day later, a police constable with less than two years until retirement was shot in the head when he answered a call for help.

Several groups claimed responsibility for the attacks. The Real IRA, which was responsible for the horrific Omagh bombing of 1998, said its gunmen shot the two soldiers. A group named Oglaigh na hEireann also took responsibility. Another organization, Continuity IRA, which is said to have organized riots among Catholics, said it had murdered the policeman.

The groups do not expect to unravel the peace accord on their own. They are hoping that they can force a security crackdown by the British or lure their Protestant counterparts, the “loyalists,” to respond with violence of their own. Fortunately, the Northern Irish people prefer to turn their backs on that madness. Thousands of people took to the streets, with Protestants and Catholics joining together to denounce the crimes. Mr. Peter Robinson, the Protestant Unionist first minister, urged fellow unionists to support the “due process of law,” an appeal for restraint by loyalist paramilitaries. Mr. Martin McGuiness, the former IRA leader who is now deputy first minister, called the dissidents “traitors to the island of Ireland” and said he would give information about the killers to the police if he had it. Times have certainly changed.

The only question now is how long the Protestants will stay their hand. The police need to find the killers and show that the system works. A number of people are already in custody but it is not clear what role they played. The British authorities have learned to avoid the heavy hand; soldiers remain in their barracks and apart from a low-profile group of surveillance experts, no new forces have been brought in.

The irony is that it is precisely the success of the peace process that made the killings possible. Formerly, no British soldier would have casually ordered out for pizza; a constable would have gone out on a call in a heavily armed vehicle, rather than an ordinary police car. It is amazing that the dissidents have not taken a life sooner.

Most experts estimate that there are only a few hundred dissidents. They look to the working-class districts, where growing numbers of impressionable unemployed youths are potential recruits. They are also attracted by the romantic image of the soldier and the status of the paramilitaries, a conception of the dissidents that is no longer in fashion. More worrisome is the belief, still a minority view, that only violence will compel the British to leave Northern Ireland.

The best counter to that is continued devolution of power, as called for in the Good Friday accord. The most important step will be strengthening local security forces and ensuring that they represent all Northern Irish. Last week, the British Parliament passed a law that passes responsibility for policing and criminal justice to the Northern Ireland government. The Northern Irish Assembly must pass a law that accepts that charge. Then the government must fund that effort and make sure that the criminal justice system is truly blind to the religion and beliefs of those brought before it. Then the Northern Irish will have taken collective responsibility for their fates, and real peace may follow.

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