AS Southeast Asia grasps for peace in Cambodia, regional leaders and diplomats are increasingly bewildered and irritated by Washington's erratic policies.A new round of negotiations to end Cambodia's 12-year civil war opened in this bawdy Thai beach resort yesterday, dominated by the question: What does the United States want? Ever since negotiated breakthroughs in Pattaya revived the deadlocked peace process in June, US officials have adamantly insisted that a United Nations-sponsored peace plan be implemented to the letter, regional diplomats say. Under that plan, the UN would temporarily assume administrative control of Cambodia until elections could be held, monitor a cease-fire, and oversee total demobilization of the rival armies. Driving American concern is the recent rapprochement between China and Vietnam, whose Communist proxies, the Khmer Rouge and the Phnom Penh regime, have kept Cambodia at war for more than a decade. On the brink of normalization in Cambodia and jittery about communism's demise in Europe and the Soviet Union, China and Vietnam are pushing to short-circuit elections and install a government of Communists in Phnom Penh, moderated for Western sensibilities by the irrepressible Prince Norodom Sihanouk. But most Southeast Asian countries as well as the US's Western allies contend the peace plan really is a framework for a settlement. As they see it, a compromise peace in Cambodia is better than no peace at all. The main hang-up is elections. In a region of autocrats and ruling generals, democracy remains at best a low priority. Even as democratic reform and populism topple totalitarianism across Europe, Asia's autarchies, with few exceptions, circle their wagons against the onslaught. What's gospel in Southeast Asia is business and making money, and that dogma inexorably drives countries in the region toward Indochina. Torn by years of war, Vietnam, Cambodia, and Laos loom as great, untapped marketplaces. "Cambodia is not just Cambodia. It is the key to the rest of Indochina," says an Asian diplomat in the region. So why does the US sit on the sidelines, regional leaders ask. Because the Bush administration, looking to reelection next year, is unsure how peace will play at home. For two years, the US has delayed political and economic normalization with war-torn Indochina. When Vietnam occupied Cambodia, Washington insisted on withdrawal. When the troops left in 1989, US officials insisted on an overall peace settlement. Yet the Bush administration's Achilles heel in the region, Asian diplomats say, remains the 2,274 Americans missing in Indochina's wars. With amazement, Southeast Asians and Westerners here watched the American hoopla over recent photographs that the powerful MIA lobby said showed captive Americans in Indochina. The story, whose limited coverage in the regional press contrasted with front-page headlines in the US, took a more bizarre twist, political observers say, when the Bush administration sent two high-level diplomats to the region to investigate. Afterward, Washington pronounced the pictures a fake. "I felt sorry for them," says a Soviet diplomat who has watched his country's own regional influence undercut by turmoil at home. "They know those guys are dead." As talks open in Pattaya, Asian participants warily watch the US, recognizing that a Washington hostage to domestic politics threatens peace prospects - and business - in Indochina.