"We are all condemned to die- not to see you again. Don't worry about me. I will follow my destiny, hoping only that you will be healthy and happy."

This stoic yet desperate farewell was written by a Cambodian mother stung by betrayal. After years of separation under the Khmer Rouge, she finally had discovered her son at the U.N. refugee camp in the Thai border town of Aranyaprathet.

Their reunion proved a painful illusion.

In the predawn hours of April 12, she, along with 1,700 other newly arrived refugees from Cambodia, were ordered by the Thai Army to board trucks for a long journey. They were to be transported, they were told, from their temporary shelter at Wat Koh to a permanent camp in Trat, 100 miles to the south.

They never arrived. Instead, they were abandoned on the frontier to begin a four - day trek through the mountainous jungles of war-ravaged Cambodia, under the armed escort and without adequate food.

"We have tried to escape from death in search of life," Toeung wrote her son. "Why have we now encountered misery even worse than ever before? I have never had enough to eat. I lost one daughter. If only I could see you every day, my sorrow would be bearable."

Toeung's letter is just one of many that have reached Thailand from this group. Refugee officials consider it authentic. All the letters speak of harsh deprivation, the grief of losing close relatives and the shock and anguish of returning to Cambodia after finally reaching safety in Thailand.

Furthermore, the 1,700 from Wat Koh are not the only Cambodian refugees to have been returned to the horror of their homeland. Informed Western refugee officials suspect that these are but a fraction of the innocent and helpless victims of the Cambodian war whom the world appears increasingly determined to shove out of sight.

The same day this group was forced to march through Cambodian jungles, President Carter's coordinator for refugees, former senator Dick Clark, was holding official talks in Bangkok. His departure statement read: "Given the confused situation in Cambodia, I was particularly gratified by Prime Minister Kriangsak Chamanan's clear assurances that the Thai government would allow Cambodian refugees temporary asylum in Thailand and that no refugees would be turned back against their will."

Thai military officials say privately that the country no longer can afford the liberal refugee policy of the past as the tides of famine and war in Cambodia threaten Thailand's own national security.

Western analysts tend to support Thai fears that several hundred thousand Cambodians could flood into Thailand by late July. Yesterday nearly 4,000 refugees - including soldiers of the deposed Pol Pot government riding nine elephants - crossed into southeastern Thai Province, according to reports from the border area.

They were disarmed and taken back to Cambodia by Thai authorities today.

More than 10,000 Cambodians have entered Thailand in the last three days, according to various reports reaching here. Some have been accomodated in camps, and others have been turned back.

Many of the refugees are women and children who said they ate wild bark and mushrooms to survive. The Vietnamese offensive against the Khmer Rouge has prevented the spring planting in western Cambodia. Both Vietnamese and Cambodian soldiers are battling for the last harvest, and even now the people are said to be eating the seeds set aside for sowing.

The Thais have been criticized for abandoning their humane policy of the past.

"But no one else is rushing to give the Cambodians a home," one military source said. "If the United Nations could guarantee to remove all the refugees that arrive here in a reasonable amount of time we would let them in. But since no other country is willing to carry these people away, we must do so ourselves."

Nor have the paradoxical lessons of Malaysia's hard - line policy that all but sealed its shores to Vietnamese "boat people" last November been lost on the Thais.

"It was not until many Vietnamese drowned just off the Malaysian coast in front of Western television cameras that the West took notice of their tragedy and responded to Malaysia's dilemma," a government official commented privately.

Although the Thai policy is to return newly arrived refugees to Cambodia, they say they intend to use more persuasion than force. Military sources say they will provide the refugees with adequate provisions plus the seeds and tools to plant a crop.

The Thais also say refugees will not be forced across the border into areas controlled by Khmer Rouge, who reportedly brutally liquidate even ordinary villagers suspected as enemy infiltrators. Refugees will be put back in safe areas, under the control either of the Vietnamese forces, or of neither side.

Since the two sides in Cambodia are fighting as much for control of population as territory, some observers her suspect that there may be some understanding between the Thais and Vietnamese to funnel the large flows of refugees into Vietnamese-controlled areas. In Hanoi's eyes this might be a friendly gesture which would help offset the safe conduct through Thai territory allowed to Khmer Rouge forces escaping the Vietnamese.

Although the 1,700 from Wat Koh have not been led into massacre, they appear to have been left in a desolate no man's land with little defense against wilderness and the war.

One group says they are under the escort of the anticommunist guerrillas of former premier Lon Nol who are operating in an insecure area, often caught in the cross fire between the Vietnamese and the Khmer Rouge.

Living here deprives me of the most basic necessities of life," a man wrote his uncle. "We are living without salt, without money. At this moment I am condemned to live with starvation."

Thailand's hard-line policy poses a dilemma for the U.N. High Commission for Refugees. Forcible repatriation violates the fundamentals of the High Commission's charter. U.N. officials here havve restricted themselves, however, to representations for fear of driving the Thais into an even more rigid posture.

Bangkok, however, prefers to deal with the International Red Cross, which provides emergency shelter and food in a war zone. Unlike the High Commission, the Red Cross can be allowed access to Cambodians crossing into Thailand, without representing a concession on Bangkok's part that they recognize them as refugees.

Many sides are working quietly for a compromise, hoping that early monsoons will relieve the military pressure on the border and reduce the threat of a massive refugee influx.

This, however, would be too late for the several thousand Cambodians already deported, unnoticed by the horrors of their homeland.

"And please tell brother Thung that I am also saying goodbye to him; most likely we will not meet each other again in this life," Toeung wrote.

"This is my own fate. I must surrender myself to it once more."