Day breaks in a banyan forest. The sound of tropical birds mingles with the soft sunlight to offer the visitor a picture of unparalleled calm and tranquility.

Yet not 20 miles from here, Vietnamese armor is rolling and Khmer Rouge guerrillas are staging hit-and-run raids against it. This is Cambodia, a nation that has known little but war for the last 10 years and is now engulfed in a struggle for existence with the far bigger and better equipped Vietnamese army.

The banyan forest, home to a secret base of the side that calls itself Democratic Kampuchea, has become a momentary island in this war. It is here that Khieu Samphan, the new cheif spokesman of the movement better known as the Khmer Rouge, has invited a small group of foreign journalists to hear his version of what has happened since the January 1979 Vietnamese invasion that drove the Khmer Rouge from power in Phnom Penh and into their current jungle bases.

Samphan, 49, who holds a doctorate in economics from the University of Paris and was once referred to by Prince Sihanouk as "my only honest minister," has replaced Pol Pot as prime minister of Democratic Kampuchea. The change is part of an effort to improve the international image of the Khmer Rouge and win widersupport for their anti-Vietnamese resistance. Pol Pot continues as leader of the guerrialla army, which still claims 50,000 troops and which apparently has survived Vietnamese "mopping up" operations during the current dry season.

With three gurrilla troop perimeters stretched out around this 20-square-mile base in northern Cambodia, and the whole area booby-trapped, mined and inlaid with the famous pungi stakes that have been the stock-in-trade of Khmer guerrillas since ancient times, Samphan is able to explain what he calls his movement's "new strategic policy" without much anxiety over Vietnamese attack.

What emerges from the two-day visit is bound to be a limited and superficial picture of the war in Cambodia. But it strongly suggests that the Khmer Rouge are by no means finished. Rather, they appear to remain a formidable fighting force, capable of fulfilling Samphan's promise of waging a "protracted people's war" against the Vietnamese and their client regime in Phnom Phenh.

The figure of 50,000 troops claimed by Samphan probably exaggerates the strength of the Khmer Rouge. It is derived from the fact that before the Vietnamese invasion, Cambodia had a standing army of more than 80,000, and that Pol Pot's forces have suffered about 30,000 battlefield casualties -- a figure cited by both the Khmer Roughe and their enemies. But in addition to the battle casualties, it must be assumed the the rampant hunger and disease of last summer and fall took a heavy toll of the Khmer Rouge forces.

Now, however, the guerrillas have been able to reorganize and regroup. They are dispersed throughout a string of base areas that stretches from Poipet to the Cardamom Mountains along the Thai border, as well as a number of key pockets deep inside the country. Able to operate in small guerrilla temas, intimately familiar with the terrain, highly disciplined and fiercely loyal, the Khmer Rouge are a military force far more potent than their numbers. And it is these factors which help explain why, after 14 months, the Vietnamese have not been able to eradicate the resistance, even with the tremendous firepower of their 200,000-man occupation army.

Although their guerrillas have proven capable of survival, the Khmer Rouge are in no position to even contemplate winning a military victory by themselves. It is here that Khieu Samphan's new policy enters into the picture.

If -- and it is a certainly a gigantic if -- the Khmer Rouge could succeed in putting the past behind them and forging a united front with Sihanouk, Son Sann and other elements of the anti-Vietnamese, anti-Communist Khmer Serei, then two important changes would take place.

First, the guerrilla forces would be augmented not only by the combined strength of the existing groups but by having a single front into which young people from the refugee camps and from the zones controlled by the Vietnamese-backed Heng Samrin government could be recruited.

Second, aid and political support from abroad, and particularly from the United States and its Western allies, could be greatly expanded.

Under those conditions, Vietnam, which is already suffering food shortages and morale problems at home as a result of its Cambodian operation, might find continuing the war simply too costly. Even if the Vietnamese did push on, the conditions for military success in a genuine national liberation war waged by the majority of Cambodians would be greatly enhanced.

But there is a long road to travel before any such scenario could possibly unfold. In the meantime, the Khmer Rouge are concentrating on their guerrilla war, taking pains to preserve their remaining forces while putting enough pressure on the Vietnamese to keep them off balance. In another six weeks the rainy season will come, and with it a setup in guerrilla activity.

Seventeen-year-old Ket Mon, in green Chinese fatigues with an ak47 rifle slung over his shoulder, offers a glimpse of the dedication that characterizes the Khmer Rouge guerrilla. "Earlier in the fighting," he says, "I was pushed by Vietnamese forces three times into Thailand. But always I came back to fight. It does not matter if I die. So many of our people have already died. What matters is to oust the Vietnamese from Kampuchea."

The commander of the 105-man unit of the Second Division that has been assembled especially for review by the foreign visitors says candidly, "I can tell you that when we fight the Vietnamese we do not fight like his, in big units with heavy weapons. When we go on missions against the Vietnamese, we go in small units of four of six, or, at the most, 12. We can send our guerrillas almost anywhere in the country. They need almost nothing, because they are used to living on only a few spoonfuls of rice a day. They can fight even if they run out of weapons and ammunition. That is the way we have trained them."

The village adjacent to Kuieu Samphan's temporary headquarters gives every indication of being the guerrilla effort. This is another characteristic of the Khmer Rouge movement: There is little distinction between civilian and solider, as every individual is seen as having some role to play in the war.

Among 600 adults in the village there are few men; it is said that most are off fighting. Women are cultivating rice and tuber crops to be sent to the front, despite the parched earth of the dry season. There are many healthy babies -- a rarity in Cambodia these days -- suggesting that the Khmer Rouge believe they will be around long enough to repopulate their depleted ranks. Boys no older than 8 or 9 hone pungi stakes with swift flashes of sharp knives.

In a makeshift school, students are taking apart radios and rebuilding them, preparing for the day when these skills will be needed to keep their guerrilla unit in touch with its comand center. A small field hospital, where a dozen patients are being treated for malaria, exhibits a wide variety of medicines from Thailand, China, Switzerland Italy and France. No longer is falling prey to disease the certain death warrant it was earlier in the fighting.

Late in the afternoon, an unannounced return to the same village brings a moe candid look at daily life among the Khmer Rouge. A large group is having dinner. It consists of three spoonfuls of rice for each person in a soup made from leaves.

A political meeting is apparently taking place. About 16 people -- perhaps one working team from the village -- are gathered in a semicircle discussing the day's events. Their leader, a beautiful young woman in the traditional black pajama-style outfit and checkered Khmai scarf, is explaining to the villagers why it is important for foreigners to visit Kampuchea. It is an object lesson in the "new strategic policy."

The extremism and rigidity of the Khmer Rouge political system has led to international condemnation of the Pol Pot regime. But now, according to Khieu Samphan, the Khmer Rouge are making an effort to break with many of the most radical and controversial policies of the past. While stopping short of directly criticizing the Pol Pot years, Samphan acknowledges that the former prime minister wa removed from his post because of opposition to his policies by many of the Khmer people as well as critics abroad.

The total evacuation of Cambodia's cities, the restrictions of freedom of movement and religion, the abandonment of currency and other policies that characterized the Pol Pot regime between 1975 and 1978 would not be repeated if the Khymer Rouge returned to power in Phnom Penh, either alone or in coalition, according to Samphan. Although he rejects charges that his government carried out a policy of genocide or mass murder, the Kemer Rouge spokesman admits that 10,000 execution took place under Pol Pot.

Asked if he really believes the past can be put aside and a new united front forged with Sihanouk after the prince's denunciations of the Khmer Rouge, Samphan says, "I know the prince. He is a patroit. . . . He knows we can fight on and, on the other hand, he also knows that he himself has no forces. This struggle will be a very long one, but on its road, eventually, all patroits will meet."

Although many observers take Pol Pot's replacement to be only a symbolic gesture, since he remains the army chief, the Khmer Rouge are serious about bringing Sihanouk's old associates and prominent intellectuals into their ranks with at least nominal status. My Man, an aging former leader of Cambodia's Democratic Party, smokes a pipe in his hammock in the base area, reading the Bangkok Post and Time magazine. "No, life was not easy fro the intellectuals before," he says. "But now there is an important place for them." Thiounn Mumm, a leading Cambodian scientist who was trapped behind the Vietnamese lines and only recently reached Thailand as a refugee, now is chief of science and technology in the reshuffled Khmer Rouge government.

The Khmer Rouge want American help very badly. Samphan does not want to discuss past disputes with the United States, such as the Mayaguez affair of American press critisim of the Pol Pot regime.

"These things are in the past," he says, "and should not be brought up."

Perhaps in an effort to clear the air with the press, Samphan now acknowledges that all 21 Western journalists who disappeared in Cambodia in the early years of the war are probably dead. As president of Kampuchea since 1975, he says, he would have known if there were any Western journalists in the country.

Samphan says he appreciates the U.S. opposition to the Vietnamese invasion of Cambodia, expressed at the United Nations. He is careful to link Soviet support for Vietnamese ambitions in Indochina with the Soviet invasion of Afghanistan. "There is a worldwide problem of Soviet expansion," he says, "and a worldwide front must be built to stop it." Stopping the Vietnamese in Southeast Asia, he argues, is a key step in preserving world peace.

At a banquet for the newsmen in their jungle redoubt, Khieu Samphan and Foreign Minister Ieng Sary fuss over their guests, explaining the intricacies of how to eat the traditional 14-course meal whose fresh ingredients have been trucked in from Thailand. The food is exquisite, accompanied by rounds of Kloster beer and Johnny Walker.

This is a meal with a message. And the message is that, even here inside Cambodia, 18 miles from a major Vietnamese position, the Khmer Rouge are confident enough to entertain the outside world with traditionally lavish Khmer hospitality.