Already a chorus of 'I told you so' is heard from those who thought it unrealistic to trust the Khmer Rouge - under whose regime in the Seventies more than 1 million people died - as a party to any peace settlement. The peace plan's foundations have been collapsing since last June, when the Khmer Rouge opted out of troop demobilisation.
The UN exercise has not been wholly abortive. The 370,000 refugees on the Thai border have been repatriated, roads have been cleared of mines and some 4.7 million eager voters have registered. The peace plan drove a wedge between the Khmer Rouge and China, its main backer, and Japanese troops for the first time took their place in a UN peacekeeping mission.
But even before the political plan was derailed, a price was being paid. All four Cambodian factions have made logging deals that have destroyed much of the country's forest cover and threaten to create long-term environmental damage; the sudden influx of foreign money has pushed up inflation and spawned rampant corruption; and with the foreign businessmen and UN peacekeepers came prostitution and Aids.
The worst blow, however, was the Khmer Rouge's recent announcement that it would not participate in the elections. The violence, including murderous attacks on ethnic Vietnamese civilians, had already started, and in the past few weeks Khmer Rouge soldiers have launched a campaign to disrupt the polls. UN officers and election workers have been killed and civilians attacked.
The Khmer Rouge has no monopoly of wrong-doing, however. Human rights activists and rival parties critical of the Phnom Penh government have been attacked, and none of the main factions has spoken out against the killings of ethnic Vietnamese.
The best hope seems to be that the elections will yield a coalition government of non-Khmer Rouge parties, strong enough to contain a long-term, low-level guerrilla war. Cambodia will have an internationally recognised government and the Khmer Rouge will be more isolated from the world than ever before.
But with the Khmer Rouge already rich on the profits of timber and gems, and neighbouring Thailand unwilling to police any sanctions, the country could revert to full-scale civil war. The Khmer Rouge is a Cambodian phenomenon and will not go away. That was always the strongest argument for trying to include it in a peace deal.
For a UN facing decisions about Bosnia, there is a moral: even with a signed peace settlement in hand and the best intentions, it cannot keep the peace, let alone make a lasting peace, in a country that does not have a national will for peace.