THE KEY POLITICAL aspect of a Northern Ireland settlement was meant to be the transfer of power over local affairs from the British Parliament in London to a new Protestant-Catholic power-sharing executive in Belfast. It was to happen by today, but it won't. The collapse of hopes that the parties would put the celebrated 1998 Good Friday peace agreement into effect is a hard blow. At the worst, it stirs the possibility of a return to the sectarian strife that has taken more than 3,000 lives, mostly the toll of paramilitaries (to the other, terrorists) on both sides.

Leading patrons of the accord, including Britain, Ireland and the United States, are emphasizing the lingering debilitating distrust among the Northern Ireland parties. The patrons fear that a "blame game" would corrode all prospect of putting the agreement back on track after a decent interval. Their caution is understandable but the hard lessons of responsibility must be drawn if the peace process is to resume.

The Protestant leader, David Trimble, a Nobel peace laureate no less, came up short. He insisted that Sinn Fein should start disarming before being admitted to political power. In this demand he had a point, as no one should have to deal with a political party with its own army. But he was rigid in not opening the political door anyway, because he had full assurances that in the absence of immediate subsequent IRA disarmament the IRA would gain no political advantage.

Unfortunately his rigidity was met on the Catholic side by a measure of obtuseness. Sinn Fein and the IRA, political and military halves of the Irish nationalist movement in Northern Ireland, should have realized it was too much to expect their suspicious would-be partners to share power with a party with its own army. At the least, Sinn Fein should have said that once it was in power it would disarm; instead it allowed to stand statements that it would not.

The situation now is that the parties are angry, posturing and out of contact, and the patrons, who have made a tremendous political investment in the peace enterprise, intend to relaunch the effort at the end of the summer. The hopeful theory is that because the fundamentals of the peace agreement have not been impaired or rejected, it will be enough to review just the matter of the details. At its simplest, it is noted that the dispute is only about "sequencing": Should disarmament precede or follow power-sharing?

The distrust between the Northern Ireland parties, or between significant elements in both of them, must perhaps be seen to be believed. They will have the summer, in any event, to contemplate the choices that now lie before them. They need a dose of realism to equip themselves for the mutual concessions essential to what ought to be their common purpose of fashioning a peace.

Some days it looks, to each other, as though they are a thousand years apart, that Protestants will never yield their monopoly of power to stay with Britain, that Catholics will never yield the option of armed struggle to unite with Ireland. But they have already come a long way to closing that gap. They must finish the job.