THE EFFORTS of the international relief agencies since 1979 have not spared Cambodia the almost certain prospect of another brutal famine in 1980. That is the stark conclusion to be drawn from the recent series in this newspaper by William Shawcross, an experienced British chronicler of Cambodia's agony. The Vietnamese, who occupy most of Cambodia, siphon off much of the food relief arriving by air in Phnom Penh, and anti-Vietnamese Cambodians and gangsters take much of what crosses by land from neighboring the Thailand. The rice crop that alone would provide Cambodians some possibility of feeding themselves remains hostage to continuing war and disorder and to difficulties encountered by the international relief agencies. The agencies are nearly out of money.

There are, theoretically, two ways to improve the situation. One is political: a big conference leading to a compromise solution between competing groups in Cambodia, between Vietnam and Cambodia and, most important, between the Soviet Union and China. But a conference is out of the question as long as Moscow and Peking give priority to their contest for influence and position in Southeast Asia -- a contest the United States can do little to moderate. Food denial, along with armed conflict, is a tactic basic to this struggle. It is literally, a murderous tactic. It is, at the same time, a standard communist practice in consolidating power over a recalcitrant peasantry. Since the Kremlin was willing to kill many millions of its own citizens in this way, there can be little surprise -- only revulsion -- that it is willing to enable clients in Hanoi and Phnom Penh to follow suit. Nor is China without its own responsibility in this regard.

The only practical way to improve the situation is for the international agencies and the countries that support them to keep helping out. The American public, no less perhaps than the international public, is upset and confused at the thought that the Vietnamese army on one side and various Cambodian soldiers on the other have prevented some of the relief from getting through to the hungriest, and that no political break-through is in sight. But much food has gotten through -- the United States, which has no standing in Vietnam, has been especially useful in facilitating aid through Thailand -- and more will get through in the future. The United States should stay in the forefront of the relief mission. To back off in frustration is to doom additional hundreds of thousands of poor Cambodians who otherwise, however miserably, might survive,