Three years ago most of the non-communist nations of Southeast Asia, with a history of squabbling among themselves, appeared to be a natural target of the insurgencies promoted by the victorious communists to the north.

Now, however, the five non-communist countries along the southern rim of Southeast Asia have dampened their old territorial disputes and are prospering. At the same time, ironically, the Indochinese states to the north have seen their victory over American arms turn sour as they have fallen into the sort of border disputes that once plagued their non-communist neighbors.

This unexpected boon for the West gives the whole area a new feeling of security achieved with little American help.

More than a decade ago, while the war in Indochina absorbed most U.S. attention, there were also growing tensions in the rest of Southeast Asia.

Indonesia threatened to attack Malaysia over territorial claims. Malaysia and Thailand sparred over guerrilla problems on their mutual border. The Philippines insisted it owned Sabah, then as now part of Malaysia.Singapore executed two Indonesian marines for bombing a building in the strategic city-state.

Today, just three years after collapse of the American effort in Indochina seemed to put the rest of Southeast Asia in the most danger, a remarkable turnabout has occurred.

Pro-West politicians here are going out of their way to cooperate with each other. Malaysian army officers train in Indonesia. Thai and Malaysian troops wage coordinated assaults on border rebels. Singapore security agents track alleged saboteurs wanted in the Philippines. Malaysian police arrest leftists wanted in Singapore.

This unexpected cooperation comes against a backdrop of a number of historical handicaps.

Eighteenth and nineteenth century colonial boundaries and migrations had cut the area into a series of often illogical pieces, putting different religious and ethnic groups uncomfortably close together. American diplomats and Asian politicians had for years shared private doubts about the ability of such nations, possessing immense natural riches and ocupying important sea lanes, to withstand communist insurgencies aided from the north.

The Southeast Asian Treaty Organization (SEATO) was created in the 1950s to meet this threat, but it included the United States as a member. Thus, to many pro-West but nationalistic Asians it looked too much like a puppet group to suit their needs.

As an experiment, they created a loose, often confusion-ridden organization, the Association of Southeast Asian Nations (ASEAN), which 11 years later seems to have provided the spark for the new feeling of unity developing here.

Founded without many high expectations in 1967, this glorified consulting and debating society for the five principal anti-communist states - Indonesia. Thailand, the Philippines, Malaysia and Singapore - has become a potent force in spite of itself. Its principal activity has been a series of conferences and meetings often so long and boring that delegates have itched like schoolboys to escape to the golf course.

Yet the ASEAN forums have allowed bureaucrats and politicians from the different countries gradually to get a clearer idea of each other's problems and develop the habit of soft-pedaling at least some of their nationalistic fears and needs.

"People talk about the ASEAN spirit," said a Malaysian diplomat. "People are afraid to score points off of other people for fear of breaking the spirit of ASEAN."

Despite the political and personal success, the organization has failed miserably so far to create anything like the Asian common market some members had hoped for. Still, the considerable material and population resources of each of the individual states give ASEAN political clout when it negotiates with Western powers like the United States.

The five nations together total about 250 million people, roughly the same at the European Economic Community that some would like to see ASEAN modeled after. The five nations control shipping routes between the OPEC countries of the Middle East and Japan and the United States. They have their own vast resources of oil, tin, timber, palm oil and natural rubber.

"Every country is growing in real terms at least 6 percent a year," said economist Narongchai Akrasanee of Bangkok's Thammasat University.

"Singapore and Malaysia are growing faster than 6 percent a year, Thailand and Indonesia about 6 percent a year, the Philippines a bit higher. What will be the sum total in 10 years? Fantastic."

That of course depends on the area's ability to withstand the inevitable ups and downs in the international market for tin, timber, oil and rubber. The ASEAN nations have tried to cushion such future blows by developing markets within the group, but so far the idea has not worked.

"They have the same raw materials to sell often, and the few light industrial goods they produce also seem to be about the same," said a Malaysian businessman. "With everyone selling toothpaste and brake shoes, everyone worries about their own share of the market."

This became clear when the five members initiated a scheme to develop five jointly owned industrial products, one in each member country, that would meet special needs of the other members and enjoy special tariff provisions. The proposed urea plants in Malaysia and Indonesia, the phosphate plant in the Philippines, the soda ash plant in Thailand and the diesel engine plant in Singapore have yet to produce anything.

Most of the projects are still nothing but papers stacked on bureaucrats' desks.

"We decided, in my opinion not quite correctly, to set up these individual enterprises in each country," said Singapore's minister of finance, Hon Sui Sen. Hon recounted the difficulties of winning agreement from the other ASEAN representatives on the board of the proposed Singapore diesel engine plant for range of products that would have some chance of bringing a profit. Other nations said the plant could not compete with their own plans for diesel engines manufactured in the profitable 1-to-500 horsepower range. Yet the market for higher horsepower engines "may be too small to be considered worthy of going ahead," Hon said.

Hon said he thought ASEAN had, however, done much to ease political tensions in the area, particularly between Singapore and Malaysia after the small, Chinese-dominated state was forced out of the Malaysia federation in 1965.

"Now that we are grouped in a community of five, we don't concentrate so much on bilateral issues," Hon said.

The same of community has also developed in small ways. Jakarta's and Manila's international airports have opened separate immigration lines for visitors holding passports from ASEAN countries. A Singapore department store recently held an "ASEAN flags" contest. Even ASEAN drivers licenses are supposed to be available soon.

None of the five members states wins very high marks for freedom of the press or freedom of speech. Each holds a number of political prisoners, counted by the thousands in the case of Indonesia.

Yet since each of the governments share similar views on the need for tight security measures, they can easily cooperate on that level also.

ASEAN diplomats regularly refrain from making any statements that suggest the five nations are moving toward some kind of military alliance. They have made some progress in easing tensions with the communist states to the north and they do not want to jeopardize that progress.Politicians in each county, however, are taking steps to make future military cooperation possible if necessary. Exchanges of military cadets and junior officers for training happen frequently. All five countries have shown interest to producing a standard M-16 as their basic infantry rifle.

The member nations have often expressed hope that their major trading partners, the United States and Japan, will realize the strategic importance of the group and award it special tariff and aid advantages. The ASEAN diplomats say they like what they have heard from Tokyo and Washington on such special aid, but they are waiting for concrete action.

Some diplomats here were miffed that the recent gathering of ASEAN foreign ministers in Washington received sparse press coverage. Others said that they expected no more than that and think the Americans will rediscover Southeast Asia after their current fascination with Africa fades.

Datuk Ali Adullah, the ASEAN secretary general based in Jakarta, remains relentlessly optimistic.

"After 10 years," he said, "people are still asking what has ASEAN done. Well, we can't show it in terms of dollars and cents. I think only the ASEAN people can feel in their hearts what has been happening. There's a feeling of ASEAN-ness.