THE CRISIS in Iran has distracted attention from what remains the greatest international tragedy since World War II, the decimation of Cambodia. There the Heng Samrin regime continues to prevent food from going to Cambodians still under the control of his equally cruel predecessor, Pol Pot, who is in the countryside hoping for a comeback. The Vietnamese whose puppet Heng Samrin is, continue to mine rice fields and conduct chemical warfare to help him out. The Soviets continue to deny relief of their own to Cambodians and to block relief offered by others. The Heng Samrin-Hanoi-Moscow combination has been able to defy the best efforts of many other nations to bring the genocide to an end.

To be sure, substantial food supplies have been sent via Thailand to the Thai-Cambodian border for Cambodians who have managed to reach that region. But this has created another problem. With the refugees have come Pol Pot guerrillas and other andti-Heng Samrin forces who have used Thai territory for sanctuary and for acquiring arms from China with which to fight on. This has produced sparks and the threat of a larger conflict between Vietnam and Thailand. It was to deter that prospect that President Carter reaffirmed American friendship for Thailand the other day.

Beyond that lies the prospect of another effort by China, enemy of both Vietnam and Vietnam's patron, the Soviet Union, to teach Vietnam a "lesson" -- that is, to attack Vietnam as China did to uncertain effect a year ago. The Chinese seem only to be pondering whether to attack before Defense Secretary Harold Brown visits Peking next month, in which case this long-sought opportunity to sharpen Chinese-American security cooperation might be put off, or after , in which case the impression would be cast that the United States had approved the Chinese blow.

But however important and troubling, these regional and geopolitical considerations must necessarily take second place to the suffering of the Cambodian people. The main thrust so far in the policy of the United States and other concerned nations is to threaten Vietnam with international isolation and the Soviet Union with shame. But Vietnam is largely beyond the reach of American policy, and the administration has felt inhibited -- too inhibited -- in publicly shaming the Soviet Union by a need to protect SALT. Moscow in fact should be constantly identified as the accessory to mass murder.It is obscene to continue huge commercial shipments of food to well-fed Russia so long as the Kremlin is an active, knowing party to the starving of the Cambodians.

It is easy enough to imagine ways in which the United States might have worked more effectively to help avert this tragedy in the past. But life is not a clock that can be turned back. The choices now are pitiably few. One is to support a United Nations appeal for an international conference to re-neutralize Cambodia under Prince Norodom Sihanouk. It is not a very good idea -- only the best one around.