PEDALING down a busy Phnom Penh street, Hen Sem braked his garbage wagon and joined the throng of curious Cambodians.Across the street in a newly refurbished estate behind a high fence, Australian soldiers unloaded a steady stream of equipment. The troops were the first arrivals of a planned United Nations peacekeeping force to oversee a fragile new peace in Cambodia. But for Mr. Hen, the soldiers mean only one thing. "Sihanouk is coming," the frail man said of the homecoming today of exiled monarch, Prince Norodom Sihanouk. "The soldiers are here because of Sihanouk." Deposed and outcast by two decades of war and revolution, Cambodia's flamboyant monarch returns to preside over a country ravaged by the past. But the prince faces an economically devastated and unstable country. With Prince Sihanouk are Western- and Chinese-backed resistance leaders, including the notorious Khmer Rouge, which for 13 years battled the Vietnam-backed regime in Phnom Penh. The way was cleared for Sihanouk's expected triumphant arrival by an Oct. 23 agreement to end the civil war and place Cambodia under a UN-sponsored peace plan. The country will be overseen by an interim council comprised of the four rival Cambodian factions and administered by the UN in the run-up to elections expected in 18 months. Still numbed by years of tumult, many Cambodians cling to Sihanouk as a symbol of a distant, tranquil past. "Many Cambodians will come out. He is still regarded as the god-king by many," adviser Veng Sereyvuth says of Sihanouk, who ruled for more than a quarter century before being overthrown in a United States-backed coup in 1970. "I think people are looking back and seeing themselves during the Sihanouk period," says Mr. Veng, who last saw Phnom Penh as a 15-year-old in 1975. "They remember how peaceful it was." But political observers caution that the former god-king will have to come down to earth quickly. Years of US bombing, the genocidal rule of the radical Marxist Khmer Rouge, and a brutal proxy confrontation between Vietnam and China has left the country in ruins. Since the August cease-fire took hold, impoverished and trigger-happy soldiers have rampaged through the countryside and refugee camps in neighboring Thailand, robbing, burning, and killing. Distrust underlies the compromise Supreme National Council, which Sihanouk referees as chairman. A senior security official warned yesterday that the regime could not ensure the safety of Khmer Rouge officials expected to arrive Sunday. And in a heavily mined country of 8 million people, the planned resettlement of 500,000 displaced Cambodians stirs growing concern. Economic reforms instituted two years ago by Phnom Penh have bettered the lot of some urban Cambodians but have left larger rural populations untouched. The UN role, its largest and most ambitious ever, also raises more questions than answers. The UN advance team and the massive peacekeeping force will soon begin the daunting task of demobilizing and disarming large parts of the four armies, enforcing the cease-fire, and striking a balance between the UN and the bickering Cambodian politicians. The next six months before the UN administration swings into full gear could be dangerous. "This is the most risky period," says a senior Soviet diplomat. "We've signed the agreement and drunk the champagne. Now, let's see what happens." Crucial to the process will be the temperamental and unpredictable Sihanouk. Khieu Kanarith, a prominent and outspoken Phnom Penh liberal, says he fears Sihanouk's return raises unrealistic expectations. "His role is very important. He could stabilize the situation or he might destabilize the process," says the former publisher who was purged from the state-run press last year by Phnom Penh's ruling communists. "With Sihanouk's coming, people think they will have good water, good food, and good salaries," he says. "But if that doesn't work out, there could be a negative reaction." Failure in prompting Cambodia's recovery could lead to a resurgence of the Khmer Rouge, which is blamed for the deaths of more than 1 million Cambodians in a brutal reign in the 1970s. The fanatic Marxists, who kept Sihanouk under house arrest but killed many members of his family, were forced to flee Phnom Penh by the invading Vietnamese in late 1978 and early 1979. Khmer Rouge officials took Sihanouk with them into exile. Although the Cambodian monarch insists the Khmer Rouge poses a greater risk in the jungles than sitting in a power coalition in Phnom Penh, the US and other Western countries are uneasy about allowing the faction's leaders back into Phnom Penh. The Khmer Rouge could do well in elections if corruption in the cities doesn't abate and development aid fails to spread to the countryside. Although many political observers predict Sihanouk will win a presidential election in the country if he runs, others contend that Cambodians remember corruption and political intrigues under his rule and say his popularity extends only so far. "The best prevention against the Khmer Rouge is to work more in the villages than in the cities," says Raoul Jenner, a Belgian specialist on Cambodia. "They are building their political credibility on the poverty."