INDOCHINA GOT BACK on the front page last week, thanks to a particularly bloody Vietnamese attack against a Cambodian encampment near the Thai border. This story is no longer compelling to Americans, who -- humiliated by defeat -- left the region a decade ago. But the fighting goes on, and -- irony of ironies -- it has taken a turn in Cambodia that Americans only dreamt of during the years of their involvement.

It wasn't just any Cambodians the Vietnamese attacked so brutally. It was the camp of a group called the Khmer People's National Liberation Front, which, despite a name that evokes the Vietcong, is a nationalist, non-communist faction of apparently increasing strength and popularity. They are one of the two principal armies fighting Vietnam's six-year occupation of Cambodia. The other belongs to the Khmer Rouge. All last year the Vietnamese said the chief obstacle to their joining in peace talks was the continued presence of the murderous Khmer Rouge. But recent events demonstrate that Vietnam is not interested in peace negotiations and that its greatest political concern is not the Khmer Rouge but this nationalistic alternative to all forms of Indochinese communism.

The KPNLF has become the "third force" -- neither communist nor corrupt -- that Americans searched for during all the years of their involvement in Indochina. Graham Greene's Quiet American died for the third force. Until the end in 1975 some American officials dreamt of its emergence to save the region, and particularly Vietnam, from communism. Now, without any military support from Washington, the KPNLF has arisen to play that role in Cambodia.

That the KPNLF has become a crucial target for Hanoi's forces is beyond question. Since they opened fire on Cambodian resistance camps in late December, the Vietnamese have virtually ignored the militarily superior Khmer Rouge in order to shell and burn the military camps and civilian villages of the KPNLF.

The KPNLF army is barely two-thirds the size of the Khmer Rouge armed forces; it is ill-equipped; it has no major foreign power backing to insure its survival above all other Cambodian factions.

In spite of this, or perhaps because of it, the KPNLF have nonetheless proven themselves in the past two or three years to be the greatest political threat to Vietnamese plans to entrench its client state in Phnom Penh. Something akin to a role reversal has occurred in this third Indochina war being fought in western Cambodia.

Whereas in the first and second Indochina wars communist guerrillas captured the mantle of independence against foreign occupation and won admiration for persevering in spite of all odds, the small KPNLF is beginning to win a similar reputation in Cambodia.

If the current war was strictly between the Khmer Rouge and the Vietnamese imposed regime of Heng Samrin, the odds would be far better for Hanoi. It would then be a simple contest between two vying wings of the same Cambodian communist party. The people of Cambodia would have the narrow choice between Pol Pot's brand of Khmer communism, which led to the death of well over one million Cambodians, or the Vietnamese-style communism now administered through the Heng Samrin regime that makes Cambodia a near-colony of Vietnam.

But Son Sann, the leader of the KPNLF, refused to leave Cambodians such limited choices. A former prime minister of Cambodia in the '60s, Son Sann organized the KPNLF around a platform espousing democratic ideals, a free, independent, non- aligned Cambodia and a sense of nationalism tied to Buddhism. Unable to get support from non-communist powers, the KPNLF has had to rely on the Chinese. Peking gives the lion's share of its military assistance to its long-time ally the Khmer Rouge and gives the leftovers to the KPNLF.

Perhaps because of this abandonment the KPNLF has surprised all sides and made deep inroads in occupied Cambodia, becoming, in many respects, the Cambodian faction the others have to discredit. Although it is small comfort, the punishing, brutal Vietnamese attacks against their camps aver the past month are a tribute to the success the KPNLF has had undermining the Vietnamese occupation.

There is proof plenty that the KPNLF rather than the Khmer Rouge are the chief political opponents at the moment of the Vietnamese occupiers and their Cambodian client -- the Heng Samrin government.

The jails of the Samrin regime are filled with KPNLF followers, or people suspected of supporting the KPNLF, not with Khmer Rouge. On the contrary, the Samrin regime has shown extraordinary leniency towards the Khmer Rouge to whom they offered a clemency program requiring no more than one month's re-education before they are welcomed back into the fold. They are seens wayward communists who need only be shown "the true path", as the Minister of Justice said, before becoming citizens with full rights in the Samrin regime.

The KPNLF, on the other hand, represent an entirely antagonistic political alternative. Over a year ago, the Vietnamese stepped in to eliminate suspected KPNLF followers in the northwest and countermanded the orders of the representative of the supposedly independent Samrin regime.

When the Vietnamese or the Heng Samrin regime are criticizing the KPNLF they say that there is nearly no difference between the KPNLF and Pol Pot's people. The Vietnamese are capitalizing on the KPNLF's entering into a loose coalition for more than two years with the Khmer Rouge and the tiny forces led by Prince Norodom Sihanouk. They were pushed into the coalition in a politically pragmatic moves urged on them by foreign powers -- China, Thailand and the U.S.

Everyone has changed sides so often in the continuing war for Cambodia it is easy to get lost in the thicket. Only one leader -- Son Sann -- has refused to completely change sides and join the enemy. Prince Norodom Sihanouk, the most famous of Cambodians, has changed sides so many times he has come to represent little more than himself and a vague Khmer nationalism. He fought against the Khmer Rouge when he was leader of Cambodia but when he was deposed in 1970 he went over to the Khmer Rouge side and used his considerable reputation to encourage Cambodians to come with him.

The Khmer Rouge rewarded him by putting Sihanouk under virtual house arrest after they came to power and announced their own government. Yet today, Sihanouk is far closer to his Khmer Rouge associates in the coalition than to Son Sann, whom he openly distrusts.

The Heng Samrin regime is led by and filled with Khmer Rouge who fought under Pol Pot and helped run his horrible regime. They joined with the Vietnamese when it was clear that they were next in line in Pol Pot's execution lists.

It is within this muddy, bloody context that Son Sann stands out even further. During the civil war, from 1970 to 1975, Son Sann refused to support either the criminally corrupt and inept regime of Lon Nol's Khmer Republic or its enemy, the communist front of the Khmer Rouge nominally headed by Sihanouk. As a result, Son Sann was threatened with arrest by Lon Nol, snubbed by the U.S. Embassy in Phnom Penh and threatened with death by Sihanouk in Peking.

A man of the 'third force' with no side to support, Son Sann returned to Paris and the life of an obscure exile whose mind is fixed on events in his homeland. When the Vietnamese were looking around for a candidate to head a puppet regime should they overthrow Pol Pot, they sent an intermediary to Son Sann to ask if he would join them. Son Sann said no, largely that two wrongs would not make a right and that he was opposed to any plans for a Vietnamese occupation of Cambodia even if it would mean the end of the monstrous Pol Pot regime.

When the Vietnamese succeeded, Son Sann moved from Paris to a base on the Thai-Cambodian border and set about transforming a tired band of refugees, newly arrived ovrseas Cambodians and young recruits into a military and political resistance force.

Through painstaking effort the KPNLF's army and political staff grew despite enemies on all sides. When Son Sann's army started in 1979 it had some 1,000 members. The Khmer Rouge, under Pol Pot, had nearly 80,000 soldiers. The Vietnamese occupation force numbered 200,000.

Yet today the numbers are revealing.

Pol Pot's Khmer Rouge, despite the vastly superior aid, number around 35,000 soldiers while Son Sann's KPNLF is thought to be between 15,000 and 20,000 soldiers.

Their appeal and propaganda reach deep inside Cambodia and into the government. KPNLF operatives have their own impressive intelligence network. When Cambodians defect from the Heng Samrin regime, dejected that the Vietnamese are still in control of the country, they join the KPNLF forces if they decide to remain involved in their country's war.

The KPNLF is overshadowed, however, by the chimera of the Third Force of Vietnam, by the extraordinary fame of Sihanouk and the battlefield reputation of the Khmer Rouge. Son Sann, a former financier who is supremely self-confident and patient, is, however, uncharacteristically modest and shy for the leader of a guerrrilla movement.

Son Sann's unlikely demeanor and his age of 73 years undoubtedly contributed to the early and consistent American position to refuse granting military aid to the KNPLF. Son Sann expected the opposite. But the U.S. would have no part in his military plans. The Carter administration decided to give its tacit support to the rearming and regrouping of the Khmer Rouge under Pol Pot. They saw no future for the KPNLF. The Khmer Rouge, on the other hand, were proven military leaders, fighters who could go head to head with the Vietnamese. Both armies benefitted from American aid to refugees along the border.

The Reagan administration continued the Carter policy although it has given greater political support to the KPNLF as it has grown in strength and influence. It was under the Reagan administration that China, the U.S., and the Association of Southeast Asian Nations (ASEAN) put unresistable pressure on Son Sann to join in a coalition with Sihanouk and the Khmer Rouge. Son Sann believes he was promised military support from the U.S. in return for joining this unholy alliance but that support never came.

But the U.S. has abdicated its military interest in the Indochina region to China. The Chinese are responsible not only for nearly all the armaments sent to the Cambodian factions fighting the Vietnamese but Peking is also the major guarantor of direct military assistance should Thailand be attacked by Vietnam in the midst of the current war.

Moreover, it appears that the lesson the U.S. believes it has learned from the last Vietnam War is that in Indochina communists are better fighters than non-communists. The Pentagon has repeatedly fought against any American military assistance to another 'third force' in Indochina, particularly not to the KPNLF.

Although that decision went against Son Sann's wishes, the results may have been to the benefit rather than the detriment of the KPLNF. If history is any judge, the KPNLF has done far better without U.S. military assistance than those resistance groups who received American aid. One need only remember the Kurds, the anti-Castro Cubans and the rebels in Angola who received direct or indirect American aid so long as an American enemy could be bled by their forces but lost that aid when Washington found their resistance inconvenient.

Some of the Cambodians in the non-communist resistance are keenly aware of the high price of American military assistance. They are veterans of the corrupt Lon Nol regime which from 1970 until its defeat in 1975, was underwritten by the U.S. Up until the last weeks the U.S. gave uncriticial support to Lon Nol despite all evidence that he was losing the war through corruption and neglect as surely as the Khmer Rouge were winning it.

For all of these reasons American military assistance to the KPNLF is not the automatic answer some have put forth. The KPNLF has emerged as the most independent Cambodian force fighting in what could be seen as a three-cornered war for Cambodia. The Khmer Rouge are discredited not only for their murderous regime but for their long-standing allegiance to and dependence on China. The Vietnamese call them a puppet of China and the Cambodian people see a germ of truth in the charge. Of course the Heng Samrin regime is regularly called a puppet of Hanoi, a charge that also sticks.

The KPNLF, the orphans of the war, cannot be portrayed as any country's client. True, the resistance does depend on the expensive goodwill of Thailand for a safehaven and dependable supply route; and without Chinese military supplies it would have languished with little change to prove its military ability. Moreover, by joining in the loose coalition with Sihanouk and the Khmer Rouge, the KPNLF has tainted its reputation. But compared to the other groups fighting in the war, the KPNLF can hardly be accused as acting as a stand-in for a major foreign power.

Son Sann is adamant that he is not interested in massive military aid, nothing that even vaguely resembles the scale of aid given the old Lon Nol regime. He is equally uninterested in the American interference that went along with the aid. Rather, he has a modest shopping list of equipment he wants shipped to his troops -- nothing more or less.

It is questionable whether the U.S. is capable much less willing to give even limited supplies without demanding a price that would diminish the appeal of the KPNLF.

As of this week, the question is of utmost importance. The Vietnamese have destroyed all of the major KPNLF camps including their headquarters at Ampil. Moreover the Vietnamese have changed tactics. Besides destroying camps, they have stationed themselves in what appear to be permanent bases smack up against the border to prevent the KPNLF from returning to Cambodia. The Vietnamese apparently want to cut off the KPNLF from their routes inside Cambodia -- routes they have used successfully to harass Vietnamese troops, organize their followers around the country, gain new recruits and circulate propagand against the Vietnamese occupation.

Those KPNLF activities have proved too effective against the Vietnamese occupiers. They hope to stamp out the non-communist resistance and leave the Cambodians with the choice of either the Khmer Rouge or their Heng Samrin regime.

The next stage is crucial. Have the non- communists suffered a military defeat that will leave them incapable of regrouping and expanding? How should the U.S. and other sympathetic powers respond without jeopardizing the KPNLF? Is the KPNLF the last gasp of the dream of a third force or has it emerged as the powerfully attractive independence fighters that somehow survive foreign occupiers as other Indochinese guerrillas before them, including those who fought with Ho Chi Minh?