WHEN the Vietnamese army overran Kampuchea early in 1979, the trickle of refugees who had been making their way across the border into Thailand since the Khmer Rouge takeover in April 1975 turned into a flash flood. First to come out following the invasion were Pol Pot's Khmer Rouge followers, who had been isolated in the mountainous western part of the country bordering on Thailand by the Vietnamese and were being starved into extinction by the invaders. A second, much larger wave of refugees followed late in 1979, made up mostly of civilians, primarily farmers, and members of the chaotically factionalized Khmer Serei resistance army.

The appearance of the starving Khmer Rouge at Thailand's border presented a moral dilemma which was never fully resolved and which proved profoundly disruptive of the international relief effort on behalf of the Cambodians. It is this effort that Mason and Brown, who participated in the operation, analyze in their excellent study. During their three years in power, Pol Pot's Khmer Rouge had closed the ancient nation to the world and inflicted on it a regime of unprecedented brutality, under which as many as one-fourth of the country's pre-1975 population died. Now they had been driven out and replaced by the far more lenient socialist regime of Heng Samrin. The new government, however, was a puppet of Vietnam, Cambodia's traditional enemy and a country that had never made a secret of its goal to consolidate all of Indochina under its rule.

How was aid to be given to the shattered country, and to the hundreds of thousands of refugees, many of them in desperate condition, gathering at the Thai border? The Hanoi-dominated People's Republic of Kampuchea (PRK) was suspicious of Western assistance programs, and, even when cooperative, was extremely limited in its capacity to handle and distribute relief shipments coming into the country. International relief agencies had reservations of their own: in 1980, as Mason and Brown point out, Vietnam had a 2-million-ton food deficit, and was feeding 200,000 of its own military personnel in Kampuchea. Who was to guarantee that material assistance sent into Kampuchea would ever reach Khmer civilians? Equally questionable was the legitimacy that assistance offered through PRK government channels would seem to confer on Vietnam's expansionist policies.

On the other hand, relief work and food distribution conducted at the border would inevitably benefit the Khmer Rouge and thus provoke the Vietnamese, who were trying to exterminate all resistance. Sufficiently angered, the PRK could close the border and shut down the entire relief program. Furthermore, the lawless, utterly fluid character of the non-Khmer Rouge border camps, many of them under corrupt military leadership and housing thriving black markets, made the very distribution of assistance a tactical problem of great complexity.

These basic problems, together with the endless conflicting moral, political and emotional viewpoints they engendered, transformed the Cambodian relief effort into an arena of cross purposes which Mason and Brown analyze from the three standpoints suggested by their title: rice, rivalry and politics. The first of three long chapters they devote to these analyses chronicles the disasters that beset the rice distribution program from its onset. The International Committee of the Red Cross (ICRC) and United Nations (UNICEF), joined by circumstance into uneasy union as the Joint Mission, were both forbidden by their charters from assisting military forces. It soon became evident that, although extremely heterogeneous in nature, most of the border camps were under resistance control. Those north of the Thai border town of Aranyaprathet were dominated by the Khmer Serei, generally as corrupt as they were undisciplined and disorganized. Those to the south belonged to the Khmer Rouge.

Khmer Serei leaders were so resourceful at using the program to enrich themselves that relief agencies despaired at finding a means of keeping assistance out of their hands. Mason and Brown contrast camps like the riotous Mak Mun, where frenzied black market trading was overseen by the gangsterlike Khmer Serei warlord Van Saren, who sabotaged attempts to get relief supplies past him to civilian refugees, with Nong Chan, where a workable distribution system was developed through the perceptiveness of unofficial ICRC delegate Robert Ashe. Ashe realized early on that nothing could be achieved unless refugee leaders were active participants in the relief operation, consulted for advice in setting up distribution procedures and maintaining order while the rice was being handed out.

Under Ashe's guidance evolved the real triumph of the Cambodian relief effort, the "land bridge" by means of which milled rice, and later seed rice, was supplied directly to "walkers" who came to the border from the interior of Kampuchea and carried it back into the country. The importance of this program for the country's recovery was apparently recognized by the PRK, which gave Kampuchean civilians easy access to the border by way of a government-operated train.

Basic disagreements regarding the role of the border program erupted into open rivalry and press warfare during the seed rice distribution program, which Mason and Brown examine in detail. While the Joint Mission, particularly ICRC, favored a ceiling on seed rice distribution at the border so as not to endanger tenuous relations with the PRK, who might view the program as an advertisement of their own inability to solve the country's problems, voluntary agencies, mosttvocally World Relief, felt that as much rice should be sent over the land bridge and into the country as was possible before planting and access to the border were made impossible by the arrival of the monsoon.

After giving an overview of the logistical problems of the operation, Mason and Brown turn their attention to the thorniest problem of all, the political ramifications of supplying aid to the Khmer Rouge camps south of Aranyaprathet. Thailand was adamant in its insistance that the Khmer Rouge be fed by the Joint Mission. Bangkok was terrified at the prospect of Vietnamese expansion into Thailand, and saw the armed camps as a protective force for its own borders. The Thais realized, however, that if they themselves did the feeding, the infuriated Vietnamese might retaliate with an armed probe across the border, as they had done in the past.

The balance and compression Mason and Brown bring to their account of the wild complications that characterized the Cambodian relief program is extraordinary. Equally so is their clearsightedness in extracting from the experience a series of management guidelines for reducing waste and exasperation in future relief operations. Refugees, they believe, must be incorporated into the planning and execution of distribution procedures, instead of being seen as helpless victims awaiting assistance from the saintly. Relief organizations must become aware of the political power they acquire as non-aligned distributors of aid, and use this clout in negotiating with host and donor governments.

In a final chapter, the severely impersonal tone which dominates the body of the book is lifted for an understated but forcefully emotional account of the authors' departure from Nong Sumet camp. The feelings of relief workers who witnessed the Cambodian tragedy are voiced here, and the emotional resonance of everything related in the earlier part of the book is released. The effect, with its implied ironies, is dramatic.

Another account of the Cambodian border operation by William Shawcross is reportedly due shortly. It will be more than interesting to see what the author of >Sideshow may add to the analysis given by Mason and Brown in their unusual and useful book.