SIX YEARS AFTER its ignominious withdrawal from Saigon, the United States is returning with growing boldness to the Asian stage. But while the theater is the same, the drama is different, nearly all the actors have been assigned startingly different roles than in the recent past -- and new questions arise about America's part in the play.

China, the implacable foe whom the United States went to war to contain in the 1950s and '60s, has been designated by Secretary of State Alexander M. Haig Jr. as "a friendly nation" which can qualify for American arms as well as American high technology and, down the road, the Reagan administration has in mind American aid. In the past several years China and the United States have forged a tacit partnership over Indochina. China is ambitious for a larger role in East Asia, a prospect which is disquieting to some of its neighbors.

Japan, in the early poast-World II decades a passive and pacifistic U.S. understudy adhering to the "peace constitution" imposed by Gen. Douglas MacArthur, is an increasingly powerful economic force in the area, and a nation which increasingly has a mind of its own. Now the United States is pressuring it to buckle on its sword again and undertake important military tasks beyond the home islands. Again, some of the Asian neighbors with long and bitter memories of Japanese attack and occupation are apprehensive.

The noncommunist Southeast Asian countries, which previously were mere specks in the big power cosmos, are increasingly important political actors. At times, as in the case of this month's international conference on Cambodia, they are taking lead parts and even writing the script for the United States and other friendly nations to follow. The contrast is startling between the 1966 Manila conference, where President Lyndon B. Johnson was the powerful ringmaster organizing and manipulating America's Asian allies, and the last month's Manila conference of the Association of Southeast Asian Nations (ASEAN), where the Asians were the ringmasters and the U.S. secretary of state an important but undominating guest.

Only Vietnam's communist state seems not to have changed its act, still being cast as villain of the piece. From the Asian perspective, Hanoi has become more villainous than before in two ways: by extending its aims beyond its original Vietnamese cause to Cambodia and Laos, and by bringing the military presence of the Soviet Union more actively to the region as Vietnam's sponsor and international protector.

The new East Asian scenario has been evolving for several years, but in recent weeks the United States part in it has advanced and become much more explicit. As is the case with other regions, the Reagan administration has yet to draw up a coherent, well consdiered policy for Asia. But as in other regions, the combination of global policy choices and a series of individual decisions of the moment add up to the rough-and-ready outline of a new U.S. stance.

The global policies of the new U.S. administration most pertinent to Asia are its over-arching anti-Soviet posture, the large increase in U.S. military spending and military emphasis, the dismantling of previous restraints on overseas arms sales and military training and the downgrading of human rights policies which had been a major factor in U.S. relations with South Korea, the Philippines and Indonesia.

The decisions of the moment included the state visits to Washington of South Korean President Chun Too Hwan and Japanese Prime Minister Zenko Suzuki, increased military assistance requests to Congress for Thailand and Indonesia, a diplomatic campaign to deny political recognition and economic aid to Vietnam, reaffirmination of diplomatic ties with Peking as wellas unofficial ties with Taiwan and, paramount in interest and importance, the decision carried by Haig to Asia to supply arms as well as more U.S. high technology to China.

In these early months the Reagan administration has followed the simple guideline of "reward your friends and press your enemies," and it has displayed little difficulty in deciding who belongs to which group. In East Asia, China is more than ever a new friend (through the handling of an old friend, Taiwan, is still a complicating factor.) And the enemy is Vietnam and its superpower sponsor, the Soviet Union.

The bold and heavy brush strokes on the Asian canvas make for clean lines and clear policy when considered in a simple "us and them" view of the region. The problem is that the simple notions may be wrong, or they may be transitory. It once seemed simple and self-evident that the United States should apply the European-centered containment doctrine to Asia, going to war to save Indochina from the expansionist blight of "a billion Chinese armed with nuclear weapons," in Dean Rusk's famous phrase. As for impermanence, the Asian scene over the four decades since 1941 has seen dramatic and rapid changes, even reversals, in national purpose, alignment and ideolgy which quite belie the rice-roots timelessness for which the region is renowned.

These thoughts occurred to me as I recently accompanied Haig on his 16-day, 27,000-mile journey across the Pacific to China, the Philippines and New Zealand to set the stage for further development of the Reagan administration's Asian policies. The settings were familiar, from the Great Hall of the People in Peking to Manila's Malacanang Palace, and some of the rhetoric echoed from an earlier time. But so much has changed, so quickly, that one was caught up short.

The most dramatic and exhilirating change was in Peking, where I last had been in 1974, before the death of Mao Tse-tung. Today China has come alive with color, opinion, personal striving and individuality which were suppressed in the stultifying orthodoxy of Mao's later years. It will be decades, if then, before this vast and most populous country can catch up with other major nations but, if the current direction is sustained, China is on its way. Given its size and its potential weight in Asia and the world, China's turn in direction is a fact of major importance.

In international terms, China today presents a paradox. Like the Reagan administration, the linchpin of its foreign policy is strong and anti-Sovietism. But unlike the Reagan administration and contrary to the impression of rising danger left by Peking's rhetoric, China has reduced its military budget about 20 percent in the past year in order to give priority to economic development.

The fiscal facts and China's shortage of foreign exchange are among the reasons why a U.S. military supply relationship with Peking may be of greater symbolic and political significance than of practical military importance. If the Chinese are going to buy much American weaponry or advanced technology, they will require an infusion of American financing. Mindful of this, the Reagan administration is preparing legislation to make possible "modest amounts" of foreign aid and subsidized financing to the regime which, only a few years ago, was commonly known as "Red China."

In the past several years China has been on a determined drive to improve its relations with a variety of noncommunist states, including the United States, Japan and Southeast Asian countries. This necessarily involved lowering its ideological voice, shifting from pressure tactics against Taiwan to a policy of peaceful engagement, and downgrading its support for communist insurgencies throughout Southeast Asia.

Nevertheless, a Southeast Asian foreign minister who asked not to be quoted by name said during Haig's Pacific tour, "We have never forgotten and we will never forget that the Chinese are communists. They have objectives of their own. Right now they are practicing united front tactics in the battle against the Russians, and they are playing down their support for indigenous communist parties in the region." But in his view, the Chinese policy is "a phrase" which may not be permanent.

In Peking, Haig spoke to Chinese leaders of the undesirability of continued support for communist insurgencies, and in Washington about the same time President Reagan reportedly told Singapore's Prime Minister Lee Kwan Yew that the United States will be closely watching Chinese actions in this respect. But the Chinese have been unwilling to end their ties with Asian insurgents, ostensibly because this could leave an opening for the Russians.

Asian concerns about China's ultimate policies and intentions, as well as parallel fears of the behavior of a rearmed Japan, contributed to an undercurrent of apprehension about the Reagan administration's policies for the region. The most urgent assurance that Southeast Asians sought on Haig's trip was that Washington does not intend to strengthen the military sinews of China or Japan in order to use those nations as surrogates, leaving smaller states to their mercies. Haig told the Asians that the United States is determined to become involved more deeply in Asia rather than to pull back again.

The immediate focus of conflict and tension is Indochina, which just keeps rolling along as a world trouble spot, confounding the desire of most Americans to forget it once and for all. Haig was a troop commander in Vietnam in 1966-7 and spent a great deal of the Nixon years working on the Vietnam war as deputy to Henry A. Kissinger. Clearly he has not forgotten. He frequently refers to "North Vietnam" while speaking of today's unified Vietnam, and he startled reporters in Hong Kong during his recent trip by referring out of the blue to the perfidy of the Vietnamese in breaking the 1973 Paris accords, which he helped to negotiate.

Reagan and Haig have repeatedly said they believe massive U.S. intervention in Vietnam failed because it was not powerful enough. They have never been reconciled to defeat. The administration's strategy in the current situation was revealed by Assistant Secretary of State John H. Holdridge in an unguarded statement to an American club luncheon while he and Haig were in Peking: "Our own history with the Vietnamese over the years suggests that they are very tough people. If you give them what they want, this doesn't make them change their policy in any way. So we will seek if we can to increase the political-economic and, yes, military pressures on Vietnam working with others and in ways which will bring about, we hope, some change in Hanoi's attitude toward the situation."

The political and economic pressures are the policies of isolating and penalizing Vietnam made public and explicity by Haig in his address to the ASEAN foreign ministers in Manila. The most likely military pressures involve U.S. assistance to the Chinese, who have borne the brunt of military opposition to the Vietnamese in a tacit division of labor with the political/diplomatic efforts of the United States since Hanoi's invasion of Cambodia two years ago, and direct U.S. aid to a "third force" element of noncommunist fighters in Cambodia.

If such a third force can be established under former Cambodian prime minister Son Sann or some other figure, there will be much pressure on the United States to back it directly. The alternative woul be a noncommunist force whose principal benefactor is communist China, thus sapping its nationalist appeal within Cambodia and among the noncommunist states of the region.

Vietnam itself seems locked ever more tightly in the Soviet embrace, and there seems little likelihood of a change in the face of mounting counterpressures from China and the West. Nevertheless, the Vietnamese recently made clear to the Thais that they wish to continue a dialogue with their non-communist neighbors, and they have continued to signal a desire for contact and sympathy from the United States and the West. Such a desire falls short, at present, of a willingness to make major changes in policy or deployment in Cambodia.

The management of this Indochina episode is central to the new situation in the region. In this, China and the noncommunist states of Southeast Asia will have as much to say as Washington. But if the United States is no longer the star actor on whom all else depends in the farflung Pacific, it still provides vitally important markets and investments and a regional security umbrella on the superpower level. And Washington, once again, is playing a key role in the Asian drama.