CAMBODIANS have this week witnessed the unimaginable - the return of Khmer Rouge officials to their capital to take a role in government. In 1975 the Khmer Rouge forced Phnom Penh's residents into the countryside to undergo "reeducation," the first step on the way to the infamous "killing fields."Khmer Rouge figures are returning to take their place on the Supreme National Council, along with representatives of the other factions that have been locked in civil war for 12 years. The council is supposed to bridge the period until national elections in 1993, in accord with the pact signed in Paris last month. Two questions are foremost: Will the Khmer Rouge be able to manipulate the electoral process through threats and intimidation? And will the international community follow through on its commitment to monitor events in Cambodia and ensure an orderly, fair transition? Reports of Khmer Rouge intimidation are already being heard. Refugees have been told that the price of remaining in camps run by the Khmer Rouge is political loyalty. Coercion of voters violates the Paris accord. The United Nations, which shoulders most of the task of rebuilding the country, will have to enforce standards of fairness. The five permanent members of the UN Security Council have led the peace effort in Cambodia; they must sustain their involvement. Why this commitment in Cambodia, which occupies the periphery of world politics? First, the moral imperative. Cambodia endured one of the century's great tragedies - the slaughter of a million or more people by the Khmer Rouge in the name of Maoist zeal. Bigger powers intervened regularly in Cambodia's affairs, adding to its tragedy. Can some of these same powers now help guide the country toward a more tranquil future? Second, a peaceful Cambodia is an investment in stability for a region where China, Japan, and the US continue to have strong interests. Third, Cambodia will be a rigorous test of the UN's ability to end a conflict and lay the groundwork for lasting peace. What the UN learns there will have many other applications.