CAMBODIA HAUNTS many Americans. Here is a country that was drawn into a larger war not of its making and was torn about. Compassion would provide all the reason necessary to sustain American interest, even if there were not also a strategic aspect. Cambodia, now occupied by Vietnam, the Soviet regional surrogate, is a pawn on the chessboard of Southeast Asia.

But what can the United States do? American choices have been severely limited by the fact that the only visible alternative to the Vietnamese has been the resistance of Pol Pot's Khmer Rouge, the (communist) force responsible for the genocidal extermination of a third or more of the Cambodian people after 1975.

Now, however, an additional alternative looms. A liberal anti-communist named Son Sann, a 69-year-old financial expert who served for 24 years under former head of state Prince Sihanouk, has been in Washington putting himself forward as a "third force." He claims to control only 3,000-plus guerrillas, as against the 30,000 or more claimed by the Khmer Rouge, but if he could get support from China, the United States or other nations of Southeast Asia, then he could go into anti-Vietnam "united front" with the Khmer Rouge without being swallowed up, or so he contends.

Son Sann was given the boost of a serious reception by the Reagan administration, which promised him food aid at the least. He fits into its general attempt to find more effective ways, with America's Asian friends, to contain Soviet power. What has to be asked, however, is where support of Son Sann might lead. The Chinese apparently support him on the theory that, however slim his chances, he can help "bleed" the Vietnamese. It would be difficult for Americans to support him in conscience, however, unless they felt, which officials apparently do not, that his prospects were reasonably good. It is not enough that Cambodians would be playing a part, again, in a larger geopolitical struggle.

It distresses many, and not just the strategists of the Reagan administration, that Cambodia is still occupied. The idea of increasing the cost to the Vietnamese has an undeniable appeal. The last four or five decades of Vietnamese history, however, do not encourage the idea that the Vietnames can be easily made to yield. There is an alternative: making a political deal with the Vietnamese. Few Americans, and certainly not this administration, would contemplate it now. Nor would we. That being the case, the administration should be very careful about starting down a road at whose end there may be only more Cambodian suffering.