Five years after the fall of Saigon, the time of testing finally may have begun for the non-communist "dominoes" of Southeast Asia.

The setting was the dawning light of the Thai border village of None Mak Mun on Monday, June 23. The sound was that of gunfire and an announcement by pith-helmeted Vietnamese soldiers that Thai civilians would not be killed, but that food would be collected.

The startled villagers fled to the surrounding rice paddies. Thailand's Army quickly challenged the intruders and drove them back across the Thai-Cambodian border with overwhelming firepower of automatic weapons, helicopter gunships and artillery.

This bloodlest and most serious encounter between Vietnam and the non-communist world since the ignominious retreat of the Americans on April 30, 1975, sent tremors through a dozen capitals and swung world attention once again to Southeast Asia.

Secretary of State Edmund S. Muskie, who flew in for a meeting of the region's foreign ministers late last week, announced a hastily approved U.S. program of accelerated arms deliveries to Thailand and denounced Vietnam's incursion as "a dangerous threat to peace." The other noncommunist states of Southeast Asia quickly closed ranks in political solidarity with Thailand.

Here in the capital of noncommunist Asia's front-line state, there is grim satisfaction at the outcome at the border and in the world at large, and suspense about developments to come.

"The Vietnamese wanted to test our guts," said Air Marshal Siddhi Savetsila, Thailand's foreign minister and secretary general of its National Security Council, in an interview.

"They know damned well they cannot conquer our country," declared Siddhi, wearing a khaki safari suit in his National Security Council office.

"But if we do not show determination that we will resist aggressors and they get away with this, then they will do it again and again. They will pressure Thailand. Our private investment will dry up. All our rich people will take their money outside the country. Our morale will be demoralized," he said.

According to Siddhi, the Vietnamese incursion was a well-planned operation based on orders dated June 10. He said the invaders were told to venture no more than two kilometers (1.2 miles) inside Thailand, and to be prepared to remain four or five days.

Uncertainty about what the Vietnamese sought to gain is shared by the chief of staff of Thailand's joint military command, Gen. Saiyud Kerdphol. In an interview, he said part of the aim might have been tactial action to strike at anti-Vietnamese Cambodians who have taken refuge at the Thai border, and part to restrict international feeding operations that benefit anti-Vietnamese guerrillas.

Saiyud expressed doubt, however, that these objectives were well-tailored to the carefully prepared and risky incursion. He speculated that, in part, the Vietnamese were sending a message of independence to their Soviet allies, who previously had promised Thailand at the highest level, on several occasions, that Thai territory would not be violated by Hanoi's forces.

According to Saiyud, it would take a 10-year Vietnamese buildup to create a serious invasion force for conquest of Thailand. In the meantime, the main dangers are internal instability, confusion and Vietnamese-fostered insurgency -- none of which are seen by Saiyud as major threats today.

It is unclear, the Thai commander said, whether the incident at None Mak Mum was a limited operation with limited aims or the beginning of a long campaign of pressure against Thailand's boarders. His main hunch is that it was a test to gauge Thai determination and defenses, with the future course depending on Hanoi's assessment of the Thai response.

From all appearances the Thais displayed both confidence and capability, attributes that -- are striking to a correspondent returning to noncommunist Southeast Asia after an absence of five years.

If it were ever true -- as popularly believed in 1975 -- that the "dominoes" would be easy pray to Vietnams's legions, it is true no longer. The Asian states seem ready and willing to fight for their continued independence -- and increasingly able, as well to stand up to the challenge.

Communist Asia has squandered the past half-decade in internecine struggles and hot wars arising from the Chinese-Soviet dispute and the clash of national interests. As a result of Vietnam's invasion and contested occupation of Cambodia and China's 1979 attack and continuing pressures against Vietnam, the predominant lines of confrontation in the region have been within the communist world rather than between the communist states and their capitalist neighbors.

The Asian "dominoes" of Thailand, Malaysia, Singaore, the Philippines and Indonesia, which organized themselves into the Association of Southeast Asian Nations (ASEAN), have used the time to build their strength and their alliances. On balance, they seem better prepared today to meet external pressures and internal stress than they were five years ago.

"In the spring of 1975, we were saying, my God, here come the Vietnamese, straight from victory over the Americans, with lots of weapons, and friends volunteering to help them from throughout the world," recalled a Malaysian diplomat."We hardly could hope for the five-year respite we have had, and nobody predicted we would have it."

Goaded by the sense of present danger, regional political leaders in early 1979 transformed ASEAN, then a "gentlemen's discussion club," into a working alliance to deal with external challenge and work out accommodations to conflict within the ASEAN group.

As the cracks and fissures in the communist landscape widened to gaping chasms and violent upheavals, differences developed among the ASEAN nations in their approaches to the new power balance:

Thailand, historically comfortable with its big neighbor, China, and historically in collision with Vietnam, forged quick ties to Peking. Although officially it sought a policy of "equi-distance" between communist powers, Bangkok has become increasingly close to Peking and estranged from Hanoi -- especially after the invasion of Cambodia moved Hanoi's troops just across the Thai-Cambodia border.

Malaysia and Indonesia, with economically powerful Chinese minorities susceptible to the siren song of Peking, tended to see China as the ultimate threat and Vietnam as a buffer state with potential for absorbing Chinese energies in the short run.

Singapore, a developed city-state dominated by ethnic Chinese, is strongly anticommunist and the home of the region's leading geopolitical strategists. Singapore opposed Vietnam strongly, largely out of concern about the spread of Soviet influence through the Hanoi connection.

The Philippines, which renewed its U.S. base arrangements and alliance, moved cautiously closer to China as Washington-Peking relations warmed. Like the Thais, the Filipinos supported the U.S. war in Vietnam with logistical facilites and troops, and thus had little in common with Hanoi.

For all the divergent viewpoints and interests, the ASEAN partners have tended to hang together in moments of challenge, reasoning that otherwise they might hang separatley.

"Even for Malaysia and Indonesia, which are more concerned about China, Peking is a potential threat rather than an immediate threat," said Singapore's deputy prime minister and veteran chief diplomat, Sinnathamby Rajaratnam.

"We may dream different nightmares about the future, but when the alarm goes off in the morning, we wake to deal with the problems at hand. And we are all strongly backing Thailand when it is confronted with the immediate threat," Rajaratnam said.

The backing for Thailand from its Asian neighbors has been rhetorical and political, rather than material. Asked in an interview if ASEAN would do more, Rajaratnam replied with a laugh, "Can you ask a sparrow to fly the Atlantic Ocean?"

ASEAN's coalition building and maneuvering between rival powers of the communist world resulted, in part, from the decline of the United States as the preeminent outside power in Southeast Asia. Immediately after the fall of Saigon, there was fear and some belief in the area that the United States would pull back from Southeast Asia altogether, leaving the "dominoes" to their fate.

The U.S. pullback has been less severe than that. The Philippines base agreements were renewed. An American military presence, although a shadow of its Vietnam-war-related peak, remains in the area. The United States still has great importance as Southeast Asia's second largest trading partner, behind Japan, and as a source of credit and technology. Moreover, the United States has taken a leading deplomatic and political role, for example in dealing with the vexing problem of Vietnamese "boat people" and other refugees.

For all this, Southeast Asia leaders and officials do not appear to look to the United States for reliable protection at the moment against military pressures. Such gestures as the airlift of accelerated U.S. military supplies to Thailand are welcome, but Asians do not mistake them for a credible security umbrella.

Thailand's leaders, as well as those elsewhere in Southeast Asia, say they can take care of themselves under present circumstances. But if the recent Vietnamese action at the Thai border proves to be a strategic rather than a tactical thrust -- or if the Thai-Vietnamese quarrel should expend in other ways toward major military conflict -- the Thais are likely to ask the United States for air support of both a combat and transport nature, according to informed Thai sources.

No such request has been made, nor has the United States volunteered such aid, according to U.S. Embassy officials.

A Thai diplomat, asked to diagram his country's relations with key countries, used a dotted line to illustrate the link between Bangkok and its former chief international sponsor, the United States.

The diplomat expressed hope that after the U.S. elections, and in keeping with challenges and events in Asia, the dotted line will be made solid again.