THE NEW agreement on Cambodia among the five permanent members of the United Nations Security Council is extraordinary. It marks a breakthrough on a major and heretofore divisive, explosive and intractable regional issue. It demonstrates the awesome capability of a Security Council working the way the founders of the United Nations intended. It opens an 11th-hour possibility of preventing the civil war that continues to rend Cambodia from becoming, again, an inferno presided over by the Khmer Rouge.

The Permanent Five's premise was that at this point it was a waste to expect Cambodians to sit down and sort out their own affairs. Acting on an idea that Rep. Stephen Solarz (D-N.Y.) had designed and Australia had put into diplomatic orbit, the Five determined to substitute a temporary U.N. presence with the authority to make the tough calls. Formal sovereignty is to shift from the contested, Hanoi-seated Hun Sen regime to an all-party Supreme National Council, which will take over Cambodia's U.N. seat from the Khmer Rouge-dominated resistance coalition. But it then falls to the U. N. to see to a transition to a new regime, disarmament, elections, human rights and guarantees of Cambodian neutrality.

These plans are unprecedented in their scope and are bound to be difficult and expensive (upward of $1 billion) to realize. But less ambitious plans for, for instance, Nicaragua-type internationally monitored elections seemed, in the eyes of a rarely united Permanent Five, unlikely either to stop the fighting or to ensure a level political playing field.

Some hard selling to the Cambodians is still necessary. Moscow, by cutting aid and retreating from empire, is applying pressure to its clients in Hanoi and Phnom Penh; Washington, exploiting its new Indochina policy statement of July, is wielding the incentive of normalizing relations with the local Communist regimes. Beijing proclaims a serious intent to rein in the Khmer Rouge, whose arms it supplies, in the context of a regional settlement. The British and French are applying their influence. The Japanese are, fortunately, working too.

It is hard to imagine that either Hun Sen or the Khmer Rouge relishes the prospect of fair elections. Prince Sihanouk and Sonn San, on the non-Communist side, may have their own reservations. But they are all being propelled into a peace process by their own war weariness and by the collective determination of outside powers. Anyone who blocks the path takes on an immense responsibility.