WITH the concurrence of the United States, one of the most brutal killer-regimes in modern history is being restored to a position of influence and legitimacy.The regime is that of the Khmer Rouge, the Cambodian communists who plundered their country's cities in the mid-1970's, destroyed institutions of learning, and sent millions to hard labor in the countryside in a bizarre restructuring of Cambodian society. In the process, members of the Khmer Rouge murdered more than a million of their countrymen and women in the so-called "killing fields," stacking skeletons and skulls in piles that were later discovered by horror-stricken investigators. The Khmer Rouge was ousted from power when the Vietnamese invaded Cambodia and installed its own government in 1979. This posed a difficult dilemma for the US. It abhorred the Khmer Rouge. But it was hardly any more enthusiastic about a Hanoi-backed government, especially as Vietnam's sponsor was the Soviet Union - a Soviet Union which was at that time mischievous and hostile. The US wanted the Vietnamese out of Cambodia, but it did not want the Khmer Rouge back in power. So it supported a strange three-party coalition, made up of two noncommunist groups and the Khmer Rouge. It supplied weapons to the noncommunists so they could fight the Vietnamese, but claimed it did not send weapons to the Khmer Rouge. As recently as a year ago, the Bush administration was declaring its "unalterable opposition" to a "return to power of the murderous Khmer Rouge." Said a high State Department official: "We've made explicit our abhorrence of the Khmer Rouge. My colleagues ... have no contact with Khmer Rouge representatives. We have never been reticent about stating that one of the central motivations underpinning our efforts at fashioning a comprehensive Cambodian settlement is ... to prevent the Khmer Rouge from return ing to power. Under no circumstances will we countenance roles for the leaders responsible for the past Khmer Rouge atrocities in Cambodia's future." Brave words, but last week the US endorsed a treaty in Paris which affords legitimacy to the Khmer Rouge. The rickety peace treaty, to be administered by the United Nations, gives the Khmer Rouge two seats on a 12-member Supreme National Council and an opportunity to participate in elections. Diplomats expressed hope that the Khmer Rouge would not maneuver its way back into power but nobody can be sure, and brows in many foreign ministries are furrowed with concern. The ruthless Khmer Rouge has not impre ssed anyone with its dedication to the democratic process. Why the American flip-flop? There are several reasons. One is that the Khmer Rouge remains the strongest militarily of the various factions warring in Cambodia; the argument is that it just cannot be excluded from a peace agreement. Another is that big-power relationships in the world have changed. The Soviet Union is now friend, if not ally, of the US and is supposed to collaborate in making the Cambodian peace agreement stick. China, the major supplier of arms to the Khmer Rouge, also helped broker the peace treaty and is supposed to use its influence to keep the Khmer Rouge in line. Vietnam is supposed to be helpful. It has withdrawn its invading army from Cambodia. It wants to be cooperative with the US, because Washington is ho lding out the carrot of a restored diplomatic relationship and American economic aid. We shall see. For the Bush administration it is a high-risk gamble. Secretary of State James Baker is a skillful compromiser who wants to see the world tidied up, with as few problems as possible left simmering for his boss, President Bush. Last week's treaty signing theoretically moves Cambodia's long civil war to an end. But if the Khmer Rouge disrupts the treaty and seizes power - especially before next year's presidential election in the US - the Bush administration will be confronted with a far bigg er crisis.