PERHAPS HALF a million Cambodians died during the Indochina war, many from American bombs and in battles in which the United States played a part. So Americans cannot be too self-righteous in respect to reports of "holocaust" and "genocide" coming out of Cambodia now. But neither should Americans be inhibited in their protests by some sense of guilt of this country's earlier contributions to Cambodia's travail.Quite the contrary. If the United States showed too little concern for Cambodia in an earlier period, then Americans should be all the readier to speak out now, for whatever it may be worth. Something terrible, if hard to see and measure, is happening to a great many Cambodian people, and outsiders with any pretension to moral worth are duty bound to speak out.

The facts are few but the outline of Cambodia's new tragedy seems to be clear enough. The communist regime that took over three years ago first drove the several million city dwellers like cattle to the countryside, a transfer it justified by the food crisis existing at the time. The regime then enforced upon the population a political discipline resulting in physical extermination on a scale evidently proportionate to that practiced by Stalin and Hitler. There are questions about the dimensions of the tragedy and the quality of the evidence describing it, but there can be no doubt that to wait for a precise, verifiable accounting before uttering a protest is unthinkable. By any standards the civilized nations of the world might apply, the new rulers of Cambodia are barbarians.

One of the problems, of course, is that the Cambodian communist leadership has cut virtually all its ties with the outside world, even with its communist neighbors, with one of which - Vietnam - it is currently waging a war. Thus it is immune to most of the usual pressures that the body of nations has available to apply against renegade states. That does not, however, absolve the international community from taking a public stand against the repression that has been going on in Cambodia. It must be confronted, discussed and investigated insofar as that is possible - and condemned, again and again.