Four years after the end of the war, farmers and children regularly lose limbs to fragmentation bombs that lie festering in the soil of this desolate valley, so long a symbol of the U.S. bombing campaign in Laos.

The people here still are caught up with survival; with rebuilding their lives, their farms and villages from the devestation of the last war.

Yet the government has already begun preparing for a new conflict. Troops are marching up the plain once again: This time, they are Vietnamese and lao soldiers heading north toward the Chinese border and what many feel is the next battle in the seemingly Indochina war.

The hundreds of Vietnamese soldiers and the Soviet advisers around the military installation near the plain are a reminder that Laos has lost its four-year attempt to remain neutral in Indochina's turbulent politics. It is now an active ally of Vietnam and the Soviet Union against China and the consequences for this country's 3.5 million people are awesome.

Laos recently sent troops-albeit only a token force-to Cambodia to help Vietnam establish a friendly government there. On its northern border, the Laotians have becom embroiled in a war of nerves with their powerful neighbors, the Chinese.

The Laotians charge that the Chinese are threatening them on the border and attempting to sabotage them by arming dissident hill tribes. Moreover, so close are the ties between Vientiane and Hanoi, there is little doubt that Laos would become involved in any renewed fighting between China and Vietnam.

Laos dramatic movement into the Vietnamese-Soviet sphere is best illustrated by the newly reported presence of Soviet military and information gathering installations on its border with China. That equipment, along with Vietnamese troops, could become an irritant to Peking and draw this small state even further into China's quarrel with Vietnam.

From the start, Laos was the quiet exception among the Communist Indochinese nations. While the Vietnamese and cCambodians fought devastating final battles to capture their cities and countries, the Pathet Lao took Vientiane in 1975 without a shot fired. The first troops to enter the city were young women who were met with flowers songs.

Then, vengeance was not on their minds. Most nations, including the United States, kept their embassies open and Vientiane remains the only city in Indochina with an American presence. Foreign aid continued from all countries except the United States and last year Laos received$160 million from Communist and noncommunist nations, international organizations and American church groups.

There were no postwar executions, according to Laotians and foreigners who were present. Many Lao who worked for the defeated government or the Americans were allowed to stay in their jobs. Soon, however, the situation began to harden.

Thousands of Lao, perhaps more than 10,000, were sent off to reeducation camps and told they would be away for three weeks, possibly a month. Most have not returned after three years. These men were not charged with specific crimes nor were they put on trial.

During the first years after the war, refugees fled Cambodia with stories of harsh brutality. Then the plight of the boat people from Vietnam captured world attention. But the Loa have been quietly fleeing their country at an increasing rate.

About 200,000 have left since 1975, giving Laos the distinction of having generated more refugees per capita than any other Asian country in modern history. And the flow has not ended. Last month another 5,000 left for Thailand.

Most of these refugees do not have horror stories to tell. Their greatest fear is the future. They say they do not understand where their country is heading.

"It's so difficult to judge how well Laos is doing or its future because the Lao have never had a chance to develop their own system without undue foreign influence," said a European diplomat in Vientiane. "Now they are becoming so much a part of the Soviet sphere, the Vietnamese sphere, that one wonders where it will end."

Laotian neutrality, a remnant of the Geneva accords of 1962, evaporated last December. Laos has five major ethnic groups; the Lao are the dominant although they are not in the majority. Neutrality had been the historic solution to hold the peoples together and keep outside powers-Thais, Vietnamese, Chinese or Americans-from dividing and conquering.

In 1976 Laos signed a peace and friendship treaty with Vietnam, roughly putting the country under the protection of the Vietnamese. This was not remarkable since Laos traditionally has needed a stronger nation to guarantee its survival.

Yet Vietnam's overthrow of the Pol Pot government in Cambodia and the Chinese invasion of Vietnam this winter swept Laos so firmly into the Vietnamese camp that Lao leaders now say they will fight alongside Vietnam should another war against China break out.

The lao have declared China to be the "number one enemy" and cited as yet uproven charges of Chinese arming northern hill tribes to sabotage the country.

"We have demanded the Chinese end their activities against us and if they do not stop . . . the Lao people will fight to protect their independence," said Khampeng Boupha, member of the Central Committee of the Lao Communist Party.

To this end, the Lao have drafted 13,000 into the Army and required citizens to attend seminars on the danger imposed by China.

Laos has other security problems beyond the Chinese. On a three-day trip in southern Laos, we were escorted by six armed guards, an unusual precaution in this country. We were told these soldiers were protecting us from "bandits," a euphemism for the 800-member strong insurgency movement that has made travel on these southern roads unsafe.

Besides acting as if it were at the brink of war, Laos appears to be at the crossroads in its new economic program, its position in the international community and its ability to prevent its people from fleeing the country in record numbers.

Loas has tried to ease the country into socialism, not force the ideology on the population and the sucess has been mixed. Buddhism, for centuries the cultural mainstay here, was not outlawed as in neighboring Combodia. It flourishes as a quasi-state religion; saffron-robed monks propagate the faith and new government policies from their pagodas.

Laos has had to operate two parallel economies: one, a private enterprise system to appease the cities, the other, state socialism to organize farms and bring cheap goods to rural areas. Always poor, the Lao economy has needed large injections of foreign aid to bring up its annual per capita income to $90, even less than Bangladesh.

With such meager results after four years, the country has decided to speed up its plan to organize cooperative farms even though this may increase the flow of refugees to Thailand.

"We cannot be sorry that these people left," said Sisana Sisane, minister of information. "We want people to stay if only they want to stay. We are creating new professionals, new farmers to replace them. The people who left did not want to work, they wanted an easy life. We hope they will come back sometimes and see that Laos is working."

Many of the foreign experts and technicians working in Laos said the problems they face in their projects have increased. They all said they hoped the country's prospect would improve but feared they would not. Highly centralized bureaucracy with few people wanting to make decisions and growing suspicions about foreigners were cited as the stumbling blocks.

For instance, last December the Lao ordered the end to any direct contract between foreigners and Lao officials outside business hours without special permission.

"Do you know what that has done to these projects? Delay, delay, delay. I can't discuss everything in an hour's business conversation and the Lao are social people who open up best with food and conversation. I have more time on my hands than any other time in my life," said a Western expert who has worked in the country for more than a decade.

The exceptions are the Vietnamese and the Soviets. Their ambassadors are the only ones in Vietnam with access to Premier Kaysone Phomvihan, the top leader of the country.

During two weeks of travel in Laos, to the north and the south, the growing influence of both nations was evident everywhere. Before, a Westerner travelling in Lao provinces would be called farang or americaine by children on the streets.

Now, the Lao school children cry "Soviet, Soviet" whenever they see a white foreigner.

The hundreds of Americans who once worked in this country either for the CIA, AID programs or the American Embassy have been replaced by Soviet Technicians and Vietnamese work crews. In Xieng Khouang Province there were large Vietnamese road-building crews. The worker-soldiers wore military uniforms and their crew leaders were armed with AK47 rifles. The Vietnamese were anxious to speak with visiting Western journalists but Lao guides discouraged it.

"Vietnam did not take over Laos as many people have said. What we have is cooperation," said Soubanh Srithirath, a high-ranking Foreign Ministry official, when asked about Vietnamese-Lao relations.

"In Vietnam there are not many natural resources, here there is much but no industry. It is because of this we are interdependent. It does not signify domination," Soubanh said.

Last year the Vietnamese gave Laos approximately $3 million in aid, according to an unpublished international aid report. The interdependence appears to be more a matter of troops, work crews and defense than economics. Laos, in fact, with its record 576,000 tons of rice last year, has been able to feed its people better than the Vietnamese.

The Soviet Union tops the lists of foreign donors and gave $28 million in aid to Laos last year, according to the same report.

Russian is fast becoming the second language at Lao schools with most students choosing it in hopes of overseas travel. The young understand the importance of the new Lao alliances as well as, if not better than, the adults.

Chimdaphone, a 14-year old novice at a Vientiane Buddhist high school, said, "Of course I am studying Russian, not English. I would like to go to Russia because I like communism. I won't go to China, I don't like Moa Tse-tung or Maoism."

"I hope to go to the Soviet Union and study with the monks there," the youth continued. "Yes, there are monks in the Soviet Union but they don't wear the same kind of robes." CAPTION: Picture 1, Vietnamese work crews are a common sight throughout Laos these days. 'Vietnam did not take over Laos . . . What we have is cooperation,' said a high-ranking official.; Picture 2, Chimdaphone, 14, would like to study in the Soviet Union some day. The people still are caught up with survival. Yet the government is preparing for a new conflict. Photos by