WHEN THE IRISH Republican Army announced its intention Thursday to abandon its armed struggle of three decades, the British prime minister, Tony Blair, correctly described the statement as a step of "unparalleled magnitude." President Bush was also correct in observing that the decision opened up a historic opportunity for Northern Ireland. In calling on their comrades to "dump arms," the IRA's leadership used a phrase with echoes in Irish history, one that seems to signify a genuine intention -- finally -- to disarm. Eight long years after the IRA agreed in principle to decommission its weapons, the movement says it is ready to let witnesses verify that it has actually done so.

Northern Ireland's Protestant unionist parties are understandably cautious. In the past six months, the IRA has carried out a major bank robbery as well as one prominent murder. In the past several years, the organization has been linked to terrorist groups in Colombia, to drug smuggling and to organized crime. Offers to decommission weapons have come to nothing in the past. And while the IRA has called on its members to "dump arms," it has neither expressed regret for its past actions nor offered to disband. This leads to an obvious question: If there is no more "armed struggle," why does the IRA need to exist at all? Could it be that the organization's underworld contacts are simply too lucrative for its members to discard?

There is still a long way to go, in other words, and the United States still has a role to play. U.S. negotiators should continue to insist that the peace process isn't "over" until the IRA has ceased to exist. U.S. influence is still significant in the province: This week's announcement came about partly because the IRA's backers, in Ireland and the United States, have a much lower tolerance for terrorist activity than they used to.

But the announcement also came about because Sinn Fein, the IRA's political wing, has at last realized that it can win far more votes by ending violence than by surreptitiously supporting it. In other words, Sinn Fein concluded that the ballot box really is more powerful than the gun. For that reason alone, the Northern Irish peace is one that the administration should seek to preserve and promote in Ireland and beyond.