Bangkok, Thailand — The sprawling encampment of Rithisen, home to 75,000 displaced Kampucheans on the Thai border, is now in the front line of battle. With the Vietnamese only five miles away, the population is packed and ready to flee into Thailand should the fighting reach them. It seems likely to be Vietnam's next target in its relentless campaign to wipe out all the resistance groups near the border.
''Hanoi's aim is to finish us off once and for all,'' says Son Sann, prime minister in the anti-Vietnamese coalition government of Kampuchea whose headquarters is at Rithisen. He has appealed to Western nations not to stand idly by while Vietnam threatens all Southeast Asia and attacks innocent civilians.
The Thai Army is now ''massively deployed,'' according to an Army spokesman, across seven miles of the most vulnerable section of the frontier, the Wattana Nakorn pass, a mountain gap which offers the quickest route to Bangkok.
The Rithisen camp stands in the center of the pass. The Vietnamese have overrun it twice before. But after a time they went away, and the population drifted back to blackened ruins to start again. The place is now a bush city with shops, light industries, schools, hospitals, and facilities for sports. The people appear to be in reasonably good health but life is Spartan.
Son Sann lives in what they call the ''brown house,'' a simple, wooden chalet. He has no office to speak of, no communications network, he gets about on a motorcycle. His life style is strictly according to the five precepts of Buddhism.
''I don't deserve to have more than other people here,'' says the septuagenarian Son Sann, who was prime minister before war came to his country in 1970.
A French-trained economist, Son Sann was minister for economy and finance when Prince Norodom Sihanouk was Cambodian head of state, and prime minister for a year in 1967-68. As founder of the National Bank of Kampuchea, Son Sann used to sign the banknotes. That made his one of the best-known names in the country.
Buddhist philosophy shapes not only Son Sann's thoughts, but also his policies. ''Buddha says nothing is permanent, everything is reversible'' is a favorite remark. And he adds, ''I don't know if Pham Van Dong (Vietnam's prime minister) knows Buddha very well, but as a Buddhist I know that nothing is irreversible.'' This refers to Vietnamese declarations that the situation in Kampuchea is ''irreversible.''
Son Sann and many of his closest followers personify the intelligentsia of a Kampuchea that has largely disappeared in more than a decade of war and genocide. Today he has 150,000 people living in areas under his control along the Thai-Kampuchea border. He claims to have many more ''silent'' supporters inside Kampuchea including some in the ranks of the administration controlled by Hanoi.
The Son Sann people on the border could not exist without humanitarian aid from outside, largely the United States, Japan, Australia, and West Germany. And Son Sann has traveled the world seeking more support. President Suharto of Indonesia recently promised him aid. In Rome the Pope told Son Sann he would pray for the Khmer people.
He has received some arms from China and Singapore. China recently promised more, and Son Sann hopes they will arrive before Vietnam launches its next onslaught. His pleas to other countries for weapons, especially for American Hellfire antitank missiles, have gone unanswered.