The mood in the brassy, normally exuberant city of Bangkok in mid-April of 1975 was one of near hysteria. Cambodia had just fallen to the Communists, and South Vietnam was soon to follow.

At the Thai Foreign Ministry, a senior official had a .38 caliber revolver and several boxes of bullets on his desk.

"I've been going to the range for target practice," he said with a nervous grin. "We've got to be prepared for the worst."

Well, the worst hasn't come.

Despite doomsday forecasts by U.S. military observers and Thai army officers, the Vietnamese tanks did not keep on rolling from Saigon to Bangkok.

The Thais and their fellow members of the Association of Southeast Asian Nations no longer believe the Vietnamese are coming. Not in tanks, anyway. The only Vietnamese entering the capitals of the ASEAN countries are diplomats and trade delegations. And the refugees, who are a problem with no visible solution.

The outcome of the Indochina wars has neatly divided Southeast Asia: three communist states and five anti-communist. The one trait they all share is the authoritarian nature of their regimes.

But, 2 1/2 years after the communist victories, the ASEAN government no longer feel seriously threatened from beyond their borders. Sporadic fighting continues on the Thai-Cambodian border, but the real threat comes from within.

In October 1976, Thailand was wracked by a brutal battle between students and police that effectively alienated a generation of the best and brightest young Thais.

The Thammasat University massacre immediately gave rise to a coup d'etat, followed by an abortive attempt last March and a second successful coup in October. Fresh signs indicate that more upheavals may be on the horizon.

If Thailand "falls," it almost certainly will collapse inward, on the heads of military officers who seem unwilling to wait for their turn in power - for their crack at the riches that Thai dictatorship has always brought.

The problems of Thailand's ASEAN partners are less forboding, but only relatively so. Malaysia, for one, has yet to resolve its age-old racial faceoff between Malays and Chinese.

Singaporeans, glutted with material success, are beginning to chafe under Prime Minister Lee Kuan Yew's imperious rule.

Indonesia, for the first time since the failed communist coup of 1965, has begun to rumble with student with Moslem extremist dissent.

Insurgents in the Philippines, bottled up for five years by President Ferdinand E. Marcos' martial law, are savagely fighting government forces in the southern and central portion of the country. Corruption within the first family has surfaced as an issue in Manila.

But the point that must be understood is that in all cases, these are internal affairs. They are within the power of each government to resolve. The domino theory, which projected the collapse of pro-Western governments in Southeast Asia once Indochina fell to the communists, has been tossed onto history's rubbish heap.

The reasons seems clear. In the first place, the Indochina wars have left the communist states spent and in no mood for aggression, at least against their non-communist neighbors.

Secondly, their war-weakened economies suffered a further battering by inadequate monsoons last season, forcing Vietnam in particular to seek rice and building materials from ASEAN and elsewhere.

The hundreds of thousands of refugees fleeing overland and by sea to the ASEAN countries have also car with them stories of horror, oppression and economic hardship. Perhaps more than anything else, these stories have impressed on ordinary people in Southeast Asia that communism is not the panacea to their own problems.

ASEAN governments are making the most of the refugees and their tales of woe. A Thai refugee official admitted quietly that local administrators often staged public meetings at which refugees described the hardships of life under communism. This practice is forbidden by the U.N. High Commission for Refugees, which finances camps for the Indochinese refugees.

Because of their own domestic crises, the Indochinese regimes have injected new vigor into establishing working relationships with the ASEAN member states, on an individual basis. As an organization, though, ASEAN remains anathema to the communists, who claim to view it as a U.S. front and a potential military alliance.

As the external threat fades, the real test facing most of the non-communist governments of the region is whether or not they are willing and capable of doing anything about the plight of their rural poor majorities.

The two ASEAN states most immediately facing crucial trials are Thailand and its neighbor to the south, Malaysia.

Since the end of the fighting in Indochina and the departure of U.S. forces, Thailand has swiftly deteriorated into the least stable regime in Southeast Asia. The explanation had little to do with ideology. Thailand's problem is its army.

Conventional wisdom has it that Thailand, to an even higher degree than its ASEAN partners, is not ready for democratic rule. Yet, in 1976, after two years of shaky civilian governments, there was good reason to believe that the next general elections would have produced viable leadership.

Dozens of tiny parties were drifting into oblivion and in that autumn, many Thai politicians believed that voters would replace aging Prime Minister Seni Pramoj and his multifaceted coalition with a relatively cohesive grouping.

Then came the bloodshed at Thamasat University on the morning of Oct. 6. The army, crestfallen and humiliated after a student-led revolution against the former military dictatorship three years earlier, seized the moment to sweep Seni aside.

The army set up Thanin Kraivichien, a former supreme court justice and a student anti-communist, as its civilian standard-bearer. Almost immediately, rumors began circulating in Bangkok that Thanin was too conservative in his attitudes on communism even for the army.

But Thanin did not play the game his military masters expected of him. He took his mission with a zealot's fervor. Committed to eradicating corruption and the narcotics trade, he began arresting officials on the take and executing drug traffickers.

Thos army officers in the power structure saw control slipping from their grasp. Those on the fringes of the inner circle feared their turn in power might never come.

This led to one inept, abortive coup attempt and, last October, to the removal of Thanin by Gen. Kriangsak Chamanan, a close friend of many U.S. military officers who have served in Southeast Asia.

Kriangsak, long alleged to be linked to the narcotics trade, has been making what one Western diplomat termed "the right noises" since emerging from his behind-the-scenes control position.

Rarely seen in military uniform anymore, he appears in conservative students, slum dwellers and peasant with visiting dignitaries, university student, slum dwellers and peasant farmers.

He has hinted that he will not punish the 18 young alleged ringleaders imprisoned since the Thammasat violence, and has appealed to the hundreds of students who have joined communist insurgents in the jungles to come home.

He has floated proposals for reversing the one-way flow of money from the countryside into Bangkok and, in foreign affairs, has taken initiatives to improve relations with the Indochinese.

Despite all this, or more likely because of it, Kriangsak is under propaganda attack by some of his military colleagues. "It's really a great pity," commented an American expert on Thai affairs who has lived in Bangkok for 14 years. "If the army can't live with Kriangsak, I doubt that they can live with anyone."

The other ASEAN governments profess to be most worried about Thailand. But some introspective observers believe that Malaysia faces an inherently more insoluble problem: racial hatred.

Roughly half of its population of 12 million consists of Malays, unofficially called "sons of the soil." More than one-third are ethnic Chinese. Thus, Malaysia is continually rubbed sore by the friction of prejudice.

The Malays accuse the Chinese of dominating the economy. The Chinese charge the Malays with controlling the government.

On May 13, 1969, the friction burst into flame in the Malaysian capital, Kuala Lumpur. The ferocity with which Malays and Chinese slashed, hacked and beat each other, burning and ransacking each other's homes and shops, was stunning.

There has been no recurrance of the week-long carnage, and government officials use this fact to refute predictions of impending touble. Yet, no evidence of a solution is in hand or even in sight.

The two communities remain insular and antagonistic. And what makes the situation so difficult, even for the most well-intentioned of Malaysian governments, is that both sides have virtually equal strengths and weaknesses.

In the remaining ASEAN countries, Indonesia, the Philippines and Singapore, the basic question is how much longer the governments believe they can afford to remain repressive. In Singapore, the question is not complicated by official corruption, as it is in the Suharto and Marcos regimes.

Corruption and repression. A deadly combination. It provoked the communists in Vietnam, and Cambodia and Laos. The lesson has been taught in Southeast Asia. But the question is whether the lesson has been learned.