SCARCELY TWO WEEKS ago, authorities of the communist regime in Cambodia were trying to convince the first non-communist reporters they had admitted since 1975 that the threat of another invasion from Vietnam (last January's faded) was serious but not all that imminent. Now, with a stunning suddenness, 100,000 Vietnamese troops, extravagantly armed with abandoned American gear, have poured across the border, captured a third of the provincial capitals and almost encircled Phnom Penh. These troops are supporting some 20,000 Cambodian insurgents, whom presumably they mean to set up in power. Reeling, the government of "Democratic Kampuchea" speaks of falling back to the countryside and continuing the war by guerrilla means.

What is going on is a deepening of a two-tiered conflict. On one tier, Vietnam and Cambodia continue an ancient ethnic rivalry, one interrupted temporarily by the common cause they made against the United States in the Indochina War and then resumed with the special bitterness that communists can bring to bear against each other. On a second tier, the two governments are acting as patrons and proxies of the Soviet Union and China, whose own rivalry now includes a struggle for power in the Asian rimlands vacated by the "imperialists." Moscow recently made Vietnam a formal ally. Peking, unmoved by the post-1975 human-rights record that has made Cambodia a pariah on the international scene, has adopted Phnom Penh. The Chinese apparently advised Cambodia, as a defense against Vietnam, to reach out to the United States.

That seems to have been the basis on which Elizabeth Becker of this newspaper and Richard Dudman of the St. Louis Post-Dispatch were invited to Cambodia last month. Nothing they found in their tightly guided tour denied, and a good bit confirmed, earlier refugee reports of mass deaths and deprivation of rights. In that sense, Cambodia, in inviting them, did not even come close to making itself worthy of the good will of the United States (or any other civilized country concerned about respect for human rights), if that was, in fact, its purpose. The journalists also found a country striving to recover from a devastating war and to construct an egalitarian society on a model of agriculture-based self-sufficiency. Too, they found a country with profound security jitters -- a condition rendered tragically real by the murder, by some kind of terrorist, of a British companion in the guest house they were sharing in Phnom Penh.

In short, much has changed since the Cambodian government led by Pol Pot defeated the American client regime in 1975, but Cambodia remains poorly placed to solicit any form of American aid in the crisis it faces now. The United States, to show its general opposition to invasions, is not resisting Cambodia's attempt to bring its complaint against Vietnam to the United Nations Security Council, where a Soviet-Chinese cat fight -- but no effective action -- is ensured. But Washington, which has diplomatic relations with neither Hanoi nor Phnom Penh, has no effective handle on their quarrel -- and no domestic pressure or license to get involved in it.

As for the confrontation growing between the Soviets and the Chinese, it must be considered as their affair. For Washington to do anything but stand by neutrally could gratuitously complicate relations with either Moscow or Peking or both. The American interest lies in doing what can be done to reassure nearby friendly countries, especially Thailand, and in supporting others' efforts to conciliate or contain the deadly struggle going on, still, in Indochina.