Lay Boonsong picked his way over a network of trenches and bunkers east of this Cambodian resistance base to a dusty clearing where two young soldiers of the Khmer People's National Liberation Front were playing boules, a French bowling game, with rough-hewn balls whittled out of chunks of wood. Beyond them, a flat and sparsely forested but heavily mined no man's land stretched away to the east.

There, about two miles away in most places and as close as 800 yards at one point, elements of the Vietnamese 5th Division reportedly are poised for an assault.

"We don't know exactly when they will attack, but we are waiting for them," said Lay Boonsong, 34, an operations officer on the camp commander's staff. In the trenches of the five-mile-long "defense line" that forms a jagged semicircle around this camp on Cambodia's western border with Thailand stood machine guns, rocket-propelled grenade launchers and other weapons.

"We are not afraid of the 5th Division because it already attacked us last April," said Hing Kunthon, 55, once a banker in Phnom Penh and now a senior official of the front. "Then we were not ready, but now we are much more prepared. We have more men and more arms."

Indeed, as something of a reward for holding out against the Vietnamese attack on Ampil in April, the Khmer front has received three deliveries of weapons from China, including one in late November, Hing Kunthon said. He said the deliveries increased the front's arsenal by about 5,000 weapons, bringing its forces up to about 18,000 men under arms. Other estimates by Thai and western diplomatic sources have put the front's armed guerrilla forces at about 12,000.

Whatever the group's strength, it is clear that the expected battle for Ampil is shaping up as a crucial one with much to lose or gain on both sides.

For both the Vietnamese and Cambodians, Ampil has a psychological value greater than its worth as home for 23,000 civilians. For it is the front's military headquarters and the key to its organization.

By contrast with previous dry-season offensives during Vietnam's six-year occupation of Cambodia, said Hing Kunthon, Hanoi's current offensive along the Thai-Cambodian border appears to have aims that are more political than military.

"They want to destroy our organization, because it is our organization that gives us strength," he said. "They also want to show the civilian population that it is very dangerous to stay with us, that they have to go abroad or to the interior."

The Khmer front, led by Son Sann, is the largest noncommunist group in an uneasy, three-party resistance coalition that has U.N. recognition as Cambodia's legal government. The strongest group, the communist Khmer Rouge, fields 30,000 to 40,000 hardened guerrillas but lacks international and domestic political support because of its brutal and destructive rule from April 1975 to January 1979, when approximately 1 million to 2 million Cambodians died from starvation, exhaustion or mass executions. The smallest faction in the coalition, the Armee Nationale Sihanoukiste under Prince Norodom Sihanouk, has about 5,000 fighters.

Although the prince, 62, a former head of state, is clearly the most popular and well-known Cambodian figure, it is the Khmer People's National Liberation Front that is "the pillar of the coalition," Hing Kunthon said.

"The Vietnamese tried to break up the coalition and didn't succeed," he said. "So now they're trying to break up the front. If they're able to destroy the front, it will leave only the Khmer Rouge, and that's their pretext for staying in Cambodia." He said that on their own, the Khmer Rouge would never be able to retain Cambodia's U.N. seat, leaving the way open for recognition of the Vietnamese-installed government in Phnom Penh under Heng Samrin.

However, according to Hing Kunthon, a member of the front's Executive Committee and a deputy chief of the general staff, it is too late now to destroy the five-year-old organization.

"The Vietnamese can destroy the camp, but they can't destroy the front," he said.

The group's organizational abilities were in evidence during an impromptu visit here Sunday. As Hing Kunthon spoke, a work detail of about 25 Khmer front soldiers in camouflaged uniforms dug a bunker for use as an underground hospital ward. The camp's adjacent hospital has a Cambodian surgical team and an operating room cooled by two Japanese air conditioners.

Camp commanders communicate via old U.S. Army radios and Japanese walkie-talkies. The camp also has schools, a military academy, meeting halls, an orphanage and neat rows of homes -- all made primarily of bamboo and thatch.

But Ampil's normally bustling residential sections were largely deserted Sunday, the civilian population having evacuated days ago to the western outskirts of the camp in preparation for crossing an antitank ditch into Thailand if the Vietnamese attack.

According to Hing Kunthon, 3,500 Khmer front guerrillas are ready to defend the camp, with another 2,000 in reserve. Ranged against them to the east along an 18-mile stretch from Banteay Chmar to Svay Chek are units of the Vietnamese 5th Division totaling 8,000 to 9,000 troops backed by armor and artillery, he said.

He said that the front now has a range of recoilless rifles, mortars, grenade launchers and even antiaircraft machine guns for possible use against Vietnamese helicopters, but that mortar ammunition was insufficient. Although the front has U.S. diplomatic backing, it must depend largely on China for arms. The Chinese supply weapons on a larger scale to the Khmer Rouge, who still enjoy Peking's political support.

Among the front's newly delivered weapons was a Chinese 82-mm mortar, the grease still on it as it sat in a foxhole near the camp's defense line.

Touring on Sunday, Lay Boonsong, the operations officer who once studied law in Phnom Penh, said the front's bunkers and trenches would enable the troops to hold their positions under Vietnamese shelling. He added that they were no longer as afraid of Vietnamese tanks since two were knocked out last week at the front's Rithisen camp to the south.

While the Cambodians here say they will fight to keep this camp, no one harbors illusions that the front's guerrillas could hold off an all-out Vietnamese assault.

"We will do the maximum to defend Ampil, but we don't want to defend it to the last man," said Hing Kunthon. While commanded by professional officers of the pre-1975 Cambodian republic, the Khmer front adheres to standard guerrilla doctrine of ceding territory to superior forces.

Yet, staring across the no man's land dotted by the stumps of trees cut down to reinforce the defense perimeter's bunkers, guerrillas like Lay Boonsong look and act confident.

"Ampil is not the same as the other camps," he said. "The Vietnamese commanders should think twice before attacking Ampil."