When 12 American-made armored personnel carriers were unloaded from a freighter here earlier this month, Bangkok's English-language newspapers made much ado.

One front-page report noted that Washington was accelerating delivery of equipment in response to the Vietnamese invasion of Cambodia. "U.S. aid Begins to Roll In," the headline proclaimed.

That headline reflected wishful thinking common in Thai government offices. In fact, the squat, tracked vehicles were among the last drops of a once torrential flow of free equipment from the Pentagon - $1.184 billion in direct grants between 1951 and 1977.

Opposition to the Carter administration to arms giveaways overseas has forced Thailand's military establishment - which has dominated government here with only brief interruptions since 1932 - to pay for almost everything it gets.

To make up for the lost aid, Thailand has begun to borrow heavily from private foreign banks and to upgrade security cooperation with fellow members of the Association of Southeast Asian Nations (ASEAN).

An estimated 10 Vietnamese divisions are operating in Cambodia west of the Mekong River, and many Thai officers believe their rich uncle has taken his leave just when he is needed most.

There is little expectation that the Vietnamese will launch an invasion of Thailand. But their presence in such numbers has underscored deficiencies in the Thai armed forces, which have not fought a major war since the 19th century and traditionally have shown more interest in political intrigue in the capital than in maneuvers in the countryside.

The Vietnamese have overwhelming superiority in numbers - an estimated 615,000 men under arms compared with Thailand's 225,000 - and in combat experience.

The Thais feel they could offset part of that disparity by continuing to modernize their weaponry.

"They desperately need a new main battle tank," noted one military observer here.

Since 1976, the Thai government has taken at least $300 million in loans from foreign private banks to buy equipment that in past years would have been provided by the United States.

Political and military cooperation with the other ASEAN members - Malaysia, Singapore, Indonesia and the Philippines - is also on the rise, diplomats noted.

In a joint communique issued after a foreign ministers meeting in Bali several weeks ago, ASEAN denounced Vietname in strong language, demanding it withdraw from Cambodia and halt the outflow of refugees, and pledges full support to Thailand.

ASEAN states - in particular, Singapore - have assured Thailand they would send arms and equipment should Vietnam invade. China, too, has publicly given a promise of aid if the Vietnamese go against everyone's predictions.

Analysts have noted new cooperation between ASEAN countries' armed forces - though always on a bilateral or trilateral basis, as ASEAN is not a formal military alliance.

Thailand, for instance, has bought helicopters and police vehicles from Singapore. The Thais want to upgrade agreements with Malaysia to further the right against Communist guerrillas operating along their common border.

Economic planners are attempting to coordinate development of defense industries.

"Should we be producing shotguns $&(WORD ILLEGIBLE and have a surplus for export," a Thai government spokesman said, "it would not be in the interests of other ASEAN countries to build similar factories."

No matter how much weaponry and cooperation the Thais obtain, serious doubts remain on their ability to repel a Vietnamese advance. Many observers here discuss the question in terms of how many days the Thai army would hold out.

This cynicism grows out of a system in which many Thai officers have more experience managing corporations and government agencies than commanding troops in the field.

Although some Thai units saw action in Korea and Vietnam, and individual soldiers fought in Laos as CIA-sponsored irregulars, the military command is heavy with men who have never fired a shot in war.

To many of them, the military is more a business than a fighting force. It runs a major bank, a chain of gasoline stations, radio and television stations, and a collection of state manufacturing corporations. Most private companies keep army or police officers on their boards to facilitate access to licensing and government contracts.

Corruption is as pervasive as in any army in Asia. Officers who earn less than $100 a week somehow find the money for luxurious houses and foreign automobiles.

Military efficiency suffers further because deployment of army units, as well as the manpower and the hardware they get, often depends upon their commanders' relations with the clique in power.

For instance, some of the best-equipped units of the First Army traditionally have been based in and around Bangkok to protect the officers running the government against their rivals elsewhere in the command hierarchy.

Still, there are signs that the unprecedented tension along the Cambodian border has forced the generals to break some old patterns in favor of national security.

Perhaps the most notable instance came last month when about 6,000 First Army troops left their barracks in Bangkok and deployed along the Cambodian frontier. It was the first time in memory that the government had allowed the units out of the city.

Despite the gestures of support from ASEAN and China, Thailand continues to believe its best friends is its former patron.

"We We know that the only country that can help us effectively is the United States," a foreign minister spokesman said. "But we don't want Gis. We want war material only."

Diplomats said the Thais see great significance in the military links they retain with the United States - $30 million in low-interest credits for the year beginning Oct. 1, and $850,000 in grants for training.

The Thais also have noted symbolic gestures from the United States, such as the ahead-of-schedule delivery of the armored personnel carriers and the raising of military credits after the Cambodian invasion from $24 million to $30 million. Should Thailand get into a serious scrap with the Vietnamese, Washington can be sure it will get the first calls for help. CAPTION: Map, no caption, The Washington Post