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.