IN THE 18 months since Vietnamese troops invaded Cambodia and pushed up toward Thailand, the large and powerfully armed governments of Thailand and Vietnam has made a generally successful effort to keep the inevitable tensions between them from getting out of hand. Their evident mutual determination to keep control has been observed with close self-interest by the United States, whose friendship and alliance with Thailand would make any clash between them a matter of special concern. Therein lies the significance of Vietnam's recent incursions into Thailand, creating the first major confrontation between these two regional powers.

There is always the possibility that the Vietnamese were merely leaping at an opportunity they had stumbled upon or exploiting one they had invented in order to humilitate the Thais. From the known sequence of events, however, another explanation appears more plausible. In mid-June, the Thai army escorted some 5,000-plus Cambodians back into Cambodia under a program of "humanitarian" repatriation overseen by the United Nations. Vietnam, however, claimed that the Thais were ushering old Pol Pot guerrillas and their families back into border areas controlled by the Cambodian resistance, all this as part of a broader Thai-Chinese campaign to block the extension of Vietnam's influence beyond its borders. There followed the Vietnamese incursions, wreaking havoc in some refugee camps and taking a toll of military casualties as well.

The Vietnamese have alternately denied that any intervention took place and described it as accidental. On the face of it, they do not seem to be looking to get into a war with Thailand at a time when they face continuing difficulties in their attempt to subdue Cambodia. At the same time, they are asking something difficult and rather inconsistent from Thailand.They are seeking, on the one hand, to open the border so that Thailand can be used as both a dumping ground for refugees generated by Hanoi's aggression and a transit and supply point for food for hungry people remaining in Cambodia, and, on the other, they are seeking to close the border, which is long and unavoidably porous, against the return of anti-Vietnamese resistance fighters.

Secretary of State Muskie had it about right when he termed the incursions part of Vietnam's "ongoing aggression." There would be no trouble if Vietnam had not invaded Cambodia in the first place. That is why it is right to call on Hanoi to withdraw, and to call on Moscow to restrain Hanoi. Lacking a suitable response, the United States has no choice but to do what it is doing -- reaffirm its commitment to Thailand and see that the Thais have the means to defend their own independence and territorial integrity.