Hatien, a port city on the South China Sea, is a ghost town lying in the burning sun. The doors and windows of the houses are closed, the stores are locked up, and nothing moves. The 30,000 inhabitants have left their city.

Tinhbien, farther east, is in ruins. The winds howl through fallen roofs, and swirls bits of old newspapers around. The walls of the houses are blackened by smoke. Tinhbien, too, is empty.

In Hoemoa, to the north, a few hundred people remained. Merchants have set up their stands beside two shelters newly covered with concrete. Soldier are digging trenches around the huts of the farmers who are still working their fields.

The road connecting the towns presents an image which eerily recalls a past believed to have been overcome: fugitives stumbling along the road loaded with heavy baskets and carrying children tied to their backs. The wounded are whimpering, crowds form around mutilated bodies, silent and helpless.

This is the border land between two Communist neighbors. Vietnam and Cambodia. Less than five years ago guerillad from both countries fought side-by-side against a common, superior enemy, the United States. They won and last, the Exhausted Vietnamese Communist promised their people "ten thousand years of peace."

But now in the last few months, the shooting and the dying have resumed, A third Indochina war has broken out, and it is the most incomprehensible one yet. Its battlefield lies between the South China Sea and the Annamite Mountain Plateau along the 660-mile border between the heavily armed Vietnamese nation of 50 million people and the 7 million Cambodians tyrannized by a group of murderous Communists.

It looks like an uneven fight and the winner seems clear. Yet, the abandoned cities, the destroyed houses and the devastated rice fields are on the Vietnamese side of the border. The grenades have hit district capitals near the border and the Vietnamese living along the entire border have been evacuated intr the interior.

This was after three major offensives by the Vietnamese army, which penetrated up to 18 miles into the neihboring country, where, by evidence of the distant roaring of guns, and despite official denials, it maintains fortified bases.

How is it possible that the Cambodians are still able to cross the border attack the villages and massacre their inhabitants? Why doesn't the Vietnamese army, which was able to conquer enemies as mighty as the French and the Americans, destroy a few thousand fanatic Khmer guerrillas?

"The situation is complicated, extremely complicated," replies Gen. Tran Van Tra, the conqueror of Saigon who how now commands the Vietnamese troops at the threatened border. He only hints at further reasons.

The roles in this third Indochina war have been reversed. The Khmer Rouge guerillas now play the same game with the Vietnamese that the Vietnamese used to wear out the Americans: nocturnal advances through impassable jungle attacks and massacres, and then quick retreats before superior forces arrive.

"It was dark. Suddenly I heard screams. I didn't know what happened I saw people run and I ran away," relates a young woman from Blay, a small settlement 3 miles from Hatien and 2 1/2 miles from the border. At 3 a.m. the Khmer Rouge attacked the village. They killed the villagers with knives clubs and sticks.

Twenty-bodies lie in a banana orchard, some of them withouts heads, without legs, with wooden stakes stuck through their abdomens and chests. The bodies are covered worms and flies. On an adjacent field lie the puffed-up remains of three water buffalo.

A mile down the road is a hut with six bodies, three of them small children. The women's bellies are slit open. The attackers cut off the head of a pet dog.

On the walls are slogans in Khmer letters: "This is our country."

Claims on the land are a reason for this cruel war. Cambodia considers vast parts of the south of Vietnam its own territory, down to the sea. Included in the territory by the Cambodians is Saigon, now renamed Ho Chi Minh City, which once, when it was founded by the Khmer, was called Prei Kor.

Above all, Cambodia fears domination of the entire Indochina peninsula by Vietnam. Hanoi's desire for a treaty on "special relations" represent for the Cambodian government a threat of colonization. They consider Laos, which agreed to such a pact, a colony of Hanoi, and indeed about 40,000 Vietnamese soldiers are stationed in among the three million Laotians.

In the beginning, the Vietnamese tried to entice their stubborn neighbors in Cambodia with concessions: They offered negotiation on new fortifications along the border and they were not even afraid to sacrifice human beings in the process, a fact which has been kept a tight secret until this day. Between April 1975 and April 1976, when today's battlefield was still called "Border of Peace and Friendship," the Vietnamese authorities forcefully sent thousands of Cambodian refugees back to their country.

Phnom Penh had required the return of Cambodians who had fled to Vietnam as a sign of goodwill on the part of the Hanoi government. It promise to treat the evacuees well.

Only Cambodians with Vietnamese or Chinese ancestor were allowed to remain in Vietrnam. The others were forcefully herded together, loaded onto trucks and delivered to the Khmer Rouge, despite fierce resistance. According to survivors who managed to flee again, all the men were killed by the Khmer Rouge.

"They tied my husband's hands on his back with a red rope and attached a big rock to it," reported Mou Sok Ka, 28, a mother of three children. "His eyes covered with a black cloth and he was ordered to kneel down in the middle of the village. The Khmer Rouge then hit his head and neck with wooden clubs until he didn't make any more sound."

"Yes, it is true, we sent the refugees back," confirms Tran Van Hieu, president of the revolutionary committee of Mochoa. "Only after we heard that the Cambodians were killing all the refugees did we let them stay - but not until thousands of Cambodians had been sent to their deaths.

Today there are thousands of new refugees in the area. In December and January the Vietnamese made several forays across the border. When they returned, most of the population followed them. Now these Cambodian refugees live in primitive camps on near-starvation rations.

"But at least we don't have to be afraid every night of being killed," said a woman with two small children. Some young men who were chosen from the ranks of prisoners and refugees are taken better care of. They now live in a camp near Vithanh, on the site of a former American military airport.

They are uniformly dressed in green fatigues and obviously well fed. Although no one talks about it, it is clear that the Vietnamese are training them to fight against the Cambodian government.

A total of 15,000 to 20,000 young Cambodians are supposedly being trained for a guerrilla war in their homeland, although the Vietnamese have had little luck so far with their fifth column. After an abortive coup in Phnom Penh, the Khmer Rouge destroyed anyone who had ever fought alongside the Vietnamese or who had ever the slightest connection with them.

Even the date of the founding party (1951) was erased. The Communist Party of Cambodia now officially exists only since 1960 - the year the current head of the Khmer Rouge, Pol Pot, began to head the party.

Vietnamese often do not have an answer when asked what will happen at the border, why the army does not finally get rid of the Cambodian invaders, why there are many points on the border where the army is not present.

"We can drive them away at any time, but we are always told to wait," says Tran Van Hieu, head of the Mochoa revolutionary committee.

Apparently the Vietnamese, who urgently need help from abroad, want to document that they are victims of aggression and not the aggressors, even if this policy costs the lives of thousands of their people. A Communist diplomat in Hanoi says, "It is only for that reason that they let the Cambodians attack."

An attack on Phnom Penh would have little result except for a negative echo in the world. The Khmer Rouge have evacuated their capital and would not lose anything if it fell. Nor would they suffer a decisive defeat. Their people and their army live in the jungle anyway.

In addition, the Vietnamese are afraid that a serious attack on Cambodia would only create the danger of a conflict with their mighty neighbor in the north, China, which probably supports Cambodia since Vietnam falls increasingly under the influence of the Soviet Union.

Yugoslav reporters, who recently went to Cambodia as the first foreign journalist allowed to go there, sighted a Chinese freighter in the port of Kompong Som. It brought mortars, rockets, 130mm guns and supposedly the Chinese have even delivered some Migs to their radical allies.

Peking's ambassador in Vientiane has already predicted that there will be a "long, long war" between Vietnam and Cambodia, which will, in the foreseeable future, prevent the war-weary Vietnamese from enjoying the fruits of their victory over the Americans. The war's cost is not merely human lives and destroyed villages.

"A single bullet costs as much as a kilo of rice. For the price of a single 105mm grenade, an entire family can be fed for two years," calculates a Vietnamese officer.