ASSISTANT SECRETARY OF STATE Richard Holbrooke gave Prince Sihanouk only a mixed report in a question-and-answer session at the Woman's National Democratic Club several months ago. Holbrooke said the prince was clearly the only Cambodian who is genuinely popular among the Cambodian peasantry. But he went on to say tht Sihanouk "right now" does not have what it would take to fulfilll his dream of returning to power and leading Cambodia back to independence and freedom: The Thais haven't liked him every since Dean Acheson helped him win the rights to a disputed temple in the 1950s at the World Court; the Vietnamese and the Russians don't like him, and the Chinese, with whom he had close relations for a long time, have been questioning his policies and prospects. 'These points remain valid today.

In his latest book -- earlier he wrote My War with the CIA -- Sihanouk disputes his detractors and says flatly: "Cambodia's last chance is Norodom Sihanouk." He argues, moreover, that the sooner he gets back on the scene the better. He pictures former premier Pol Pot's Khmer Rouge guerrilla force, still widely recognized as the lawful government of Cambodia, as a continuing obstacle to a neogitated solution as long as it retains Peking's support. "The longer China delays dropping the Khmer Rouge, the stronger the Vietnamese and Soviet position in Kampuchea will become," he says.

Sihanouk arrives at the Sihanouk solution by the proces of elimination: The Russians will never allow the Khmer Rouge, China's allies, to come back into power. China will never allow the present puppet government of Heng Samrin, allied with Hanoi and Moscow, to consolidate its rule. Neither China nor the Soviet Union would go to a third Geneva conference. The Western powers are too much occupied elsewhere to think of sending in a peace force. Direct Chinese intervention, along the lines of its intervention in the Korean War in 1953, would risk major Soviet reprisal. The prince concludes: "There is really only one way out of this vicious circle; sending Norodom Sihanouk to Hanoi as Cambodia's plenipotentiary ambassador entrusted with negotiating a settlement with the Democratic Republic of North Vietnam, a settlement honorable for all concerned."

His later remark, on his return to Peking after a three-month tour of the West, that the Cambodians were better off under Heng Samrin than under Pol Pot and that he would like to return home, was public bid to play that role as an envoy of the Heng Samrin government and try to bring an end to an occupation and civil war that are a burden to Vietnam as well as a continuing disaster for Cambodia.

Besides making his own case, Sihanouk includes useful history and analysis of the forces that have been tearing at Cambodia for the past 35 years, as well as some fresh facts that help explain events in that troubled land.

He credits Pol Pot and his associates with raising a strong and courageous army that achieved remarkable successes, although he ridicules their claim that they defeated the Americans, the South Vietnamese and the forces of Lon Nol with only "kitchen knives, bows and arrows, and a few old rifles." Actually, North Vietnamese artillery, tanks and infantry divisions helped the Khmer Rouge win many of the battles that they boast of having won single-handedly, says the prince.

Having said that however, Sihanouk shows how the Khmer Rouge leaders recruited the poor, the ignorant and the neglected and instilled a hatred for anyone who was well housed, clothed and fed and anyone who was educated. This provides an explanation for the mass killings during Pol Pot's rule and lends some credence to Foreign Minister Ieng Sary's contention that the killings were the result of uncontrollable vengeance rather than part of a deliberate plant to reshape Cambodian society by killing off the upper and middle classes.

But such an army, however effective in war-time, turned out to be a vicious and self-destructive administrative force in time of peace. Sihanouk says 12-year-olds were mobilized as soldiers who would have no recollection of the good life under Sihanouk, While under house arrest in Phnom Penh, he says, he watched them torturing dogs, cats, monkeys and geckos as part of their training in cruely in preparation for fighting the Vietnamese.

Above all, says Sihanouk, the Khmer Rouge won the five-year war with the seizure of Phom Penh on April 17, 1975, by tricking everyone into believing that helping the Khmer Rouge was the best way to get Sihanouk back in power. And he says the United States lost the war because President Richard Nixon and others underestimated Sihanouk's strength and popularity and permitted his overthrow in the coup of March 1970.

The Khmer Rouge lost to the Vietnamese in the lightening invasion that began Christmas Day, 1978, he says, because they had the megalomanic idea that they could take on Vietnam and regain the lost Cambodian territories, and because the Vietnamese had done a far better job of ingratiating themselves among the Cambodian people. Another factor he mentions was massive Soviet military aid and Soviet encouragement -- provided, he says, because when the Khmer Rouge forces entered Phnom Penh in 1975, they shot up the Soviet Embassy and drove the Soviet Diplomats, hands tied behind them, through the streets of the French Embassy, the concentration point for foreigners.

Sihanouk, although his hair is graying, still is pudgy, still giggles a lot, and is hard for Americans to take seriously. But, as his new book shows once again, he knows his own country, perhaps better than anyone else. If his plans seem improbable, the alternatives seem unbearable. He has stated a persuasive case for himself.