BURIED IN the circuitous history of Washington-Peking relations which are now officially wreathed in a glow of "friendship" is a hidden chapter of warfare which occurred out of camera range of the most publicized conflict in modern times, the American war in Vietnam.

Comparatively few Americans have a glimmering that the United States and China even spilled a drop of each other's blood in Vietnam. Not only did they clash physically and violently on numerous occasions; their encounters produced hundreds, if not thousands, of casualties on both sides. american and Chinese pilots fought at least a dozen air battles; Chinese-manned antiaircraft batteries shot down scores, if not hundreds, of American planes over North Vietnam; American bombers regularly pounded Chinese-operated air defenses in that nation.

There may even have been a brief but furious ground battle between Americans and Chinese at an installation known as Son Tay, 23 miles west of the North Vietnamese capital of Hanoi.

Chinese personnel possibly were the secretly reported, unidentified dozens of "large orientals" who were caught by surprise and slain while fighting in their underwear by equally startled U.S. Special Forces who landed "by mistake" on top of them in the quarter-moon darkness of a November night in 1970. If those victims of the long-concealed portion of the Son Tay raid were Chinese, they too should be added to the never officially admitted casualty lists. For on the existing official record, no "confirmed" firefight between Americans and Chinese ever occurred. In the knowledge of most Americans, all that happened at Son Tay was that an attempted rescue of American war prisoners found only embarrassingly empty prison cells.

Where does this information come from? And why is it relevant now?

It comes from interviews with former U.S. officials, from several published but not widely circulated sources and from official American documents declassified after the end of the war. This information, only summarized in this article, may in turn by only fragments of the full record of undisclosed combat over a decade or more.

As to why it is relevant there are at least two answers: One is the need for the fullest public record for a nation which must rely on public support to sustain its policies. Furthermore, the People's Republic of China is currently engaged in an extraordinary act of public soul-searching of the past, with confessions of grievous error, to justify and sustain its attempts to regenerate the staggered Chinese social and governmental system of Mao Tse-tung.

That process overlaps the time frame of what might be dubbed the third "secret war" in Indochina, between China and the United States -- far more clandestine than what has been characterized as the once "secret war" in Laos conducted by the Central Intelligence Agency and Laotian tribesmen, and the subsequent "secret war" of bombing North Vietnamese installations in Cambodia, beginning in 1969, with a falsified public record.

It is not known if the PRC will ever reveal its full version of what it contributed to Communist Vietnam when China was a close ally of that nation, which it invaded in February 1979, "to teach it a lesson." China's present leader, Deng Xiaoping, wryly stated in an interview last year, "Do you know the amount of help we gave to the Vietnamese in those years [of the American-supported war against the North]? Twenty billion dollars. For a country as poor as China, it is much."

Deng made no mention of the price that China paid in lives to support North Vietnam; nor have Chinese leaders ever discussed publicly that for their own mutual self-interest neither China nor the United States ever officially have confirmed that they engaged in various levels of combat in Vietnam.

For a nation of a billion people, the number of Chinese casualties in the Vietnam war was not great. According to American sources, a Chinese official told a French military officer in recent years that Chinese casualties in Vietnam totaled 2,000 -- presumably meaning deaths, as distinguished from dead and wounded. The number of casualties inflicted on American forces by Chinese military personnel in the Vietnam war is unknown and unknowable because Americans only rarely knew if they were being fired at by Chinese, Russians, North Koreans or anyone else, as distinct from North Vietnamese or Vietcong.

What is known, according to authoritative American sources, includes the following highlights:

Item: During the 1960s the United States and China on numerous occasions engaged in aerial combat over North Vietnam and over the China-North Vietnam border. According to public Chinese claims, their pilots shot down seven American military aircraft during the Vietnam war between 1965 and 1967, and damaged two others. Loss of two of these planes was "confirmed" by official American sources and damage to two planes was described as "possible." Peking said it lost one Mig17 to American aircraft over China on May 12, 1966; the United States said nothing. These details are recorded in a 1975 book unknown to the general public ("The Chinese Calculus of Deterrence"), written by one of the most authoritative U.S. specialists, Prof. Allen S. Whiting of the University of Michigan. He was director of research and analysis for the Far East in the State Department from 1962 to 1966 and deputy U.S. consul general in Hong Kong, the prime U.S. listening post on China, from 1966 to 1968. Much of his material is based on "information available to the author from officially compiled data."

Item: Between 1965 and 1968, approximately 50,000 troops of China's People's Liberation Army were stationed in North Vietnam, directly supporting that nation's military operations and safeguarding China's security interests, Whiting reported. These troops included a large Chinese force which maintained a major base complex at Yen Bai in the northwest sector of North Vietnam, with a 5,000-foot runway, nearly 200 buildings and antiaircraft guns mounted on railroad tracks which permitted the weapons to be moved into caves. Chinese antiaircraft batteries at many locations regularly fired on American air missions and were bombed in return. China also maintained large numbers of engineer and transportation personnel in North Vietnam to maintain bridges and roads. Some became casualties.

Item: North Vietnamese aircraft which engaged in combat with American aircraft frequently took off from, and returned to, specially constructed airfields on the Chinese side of the North Vietnamese border. Chinese sources these days confirm this without hesitation, as evidence of the support given to North Vietnam in those years. According to American sources, Chinese and North Vietnamese communications and radar systems were designed to be mutually reinforcing in the cross-border region.

It is far less of a secret that American strategy in the Vietnam war was designed to avoid open conflict with China, and that China on numerous occasions was genuinely fearful that the war would be extended to China. That is documented in detail in numerous secret American documents "leaked" to the public in 1971 in the "Pentagon Papers" furor. According to Whiting, the mutual interest on both sides to avoid disclosing that the two nations were sometimes direct combatants reached the point that an American plane which entered Chinese airspace was pursued a dozen miles into North Vietnam and shot down by Chinese fighters with the victory attributed by Peking to Hanoi -- a claim which the United States knew was untrue but chose not to dispute.

President Johnson in private boasted repeatedly of his administration's ability to indtroduce what ultimately became more than 500,000 American troops in South Vietnam, and to conduct a calibrated, escalating air war against North Vietnam without touching off an overt American-Chinese war.

In Johnson's favorite sexual analogy, the important distinction in the strategy was between seduction and rape: By incrementally tightening the pressure on Hanoi's leadership, North Vietnam and its Chinese and Russian allies were being given no justification for violent outcry about an expanding war; China could not scream, "Rape!"

There was a high price for seduction, however, in the judgement of American military commanders.

In order to avoid crossing the threshhold which could bring China openly into the war, American "buffer zones" which varied in width over the war years kept, U.S. aircraft away from the sensitive Chinese-North Vietnamese border. As a consequence, China was able to build up large stockpiles of war supplies for North Vietnam in the buffer zone, and North Vietnamese aircraft pursued by American fighters had a corridor of sanctuary to airfields over the Chinese border.

As the conflict in Vietnam intensified, China in 1966 was plunged into chaos by Mao's Cultural Revolution, now offically acknowledged by Mao's successors as a "catastrophe." With its entire society in turmoil, China was then more vulnerable to foreign attack than it had been in a generation. From a cold-blooded strategic standpoint, the country was a tempting target for its most powerful enemies -- the United States and, by then, the Soviet Union as well.

At alternate times, although neither nation would conceive of admitting it officially, the United States and the Soviet Union each have contemplated doing to China what Israel recently did to Iraq: destroying its nuclear installations. The American consideration is well documented on the authoritative, though unofficial, record; Moscow, far more cautious about what it discloses, has never gone beyond hints that it considered "taking out" China's nuclear war-making capacity.

One of the enthusiasts for the elimination of China's then embryonic nuclear bomb facilities was the late Robert F. Kennedy, when he was attorney general. His brother, the president, also spoke in private about the possibility of such action in the early 1960s, before China tested its first atomic weapons.

Following a conversation with President Kennedy, the late Stewart Alsop wrote in The Saturday Evening Post that "a surgical strike" against China's gaseous diffusion plant at Lop Nor was under serious consideration. Stewart's brother, columnist Joseph Alsop, an intimate of the Kennedys, wrote in 1973 that "President Kennedy, who took an exceedingly dark view of the Chinese nuclear program, had ordered exploration of the idea of destroying that program in some sort of collaboration with the Soviets."

But both Presidents Kennedy and Johnson concluded that such an act raised the risk of touching off World War III. By the outset of the Nixon administration, however, the Soviet Union was alarmed about China's growing nuclear stockpile. Either for psychological warfare purposes or for genuine intent, the Kremlin sent out numerous diplomatic feelers to learn if the Nixon administration would give tacit consent to a "preventive" Soviet attack to destroy China's nuclear capacity.

Just at that time, however, President Nixon and his national security adviser, Henry A. Kissinger, were secretly headed in exactly the opposite direction: to execute a historic turnabout in American-Chinese relations which would line up China with the United States as global counter-weights to the Soviet Union.

On the scale of these geopolitical maneuverings, what occurred at Son Tay in North Vietnam on Nov. 21, 1970 -- in the midst of the Nixon administration's secret moves to open a "back channel" of communications to Peking behind the backs of its own bureaucracy -- is of relatively minor significance. But the secrecy which surrounded that particular incident is indicative of the entire climate of the times.

More was at stake at Son Tay than even the tantalizing prospect of sending a commando unit "right into Hanoi's goddam backyard" and plucking out of their prison cells some 70 American POWs. The Nixon White House was desperately in need of a moral and psychological victory in order to sustain the entire furiously controversial war effort. The open expansion of the war into Cambodia in April 1970 to strike on the ground at Vietnamese communist "sanctuaries" across the border had set off an uproar on university campuses and shattered what remained of the crumbling American consensus for support of the war.

According to an exhaustive but inescapably incomplete 1976 postmortem on the Son Tay venture, "The Raid," by Benjamin F. Schemmer, editor of the Armed Forces Journal, Nixon was given assurances of success in an Oval Office briefing by Adm. Thomas H. Moorer, chairman of the Joint Chiefs of Staff, three days before the raid.

The operation did succeed -- brilliantly -- as a high-risk military venture, former defense secretary Melvin R. Laird and other senior Pentagon officials insisted in interviews last week. There were just two surprises: No American prisoners were found at Son Tay; they had been moved perhaps 4 1/2 months earlier. And there was one other totally unanticipated "glitch": 4Part of the raiding party did not land in the dark of night at the Son Tay prison compound, but at a secondary school compound just 400 meters south which resembled it from the air. That error was not disclosed in the national controversy that welled up over the wisdom of the raid.

It was also minimized in the since declassified secret after-action report signed by the commander of the Joint Contingency Task Group, then Brig. Gen. Leroy J. Manor of the Air Force, in which he said: "I can unequivocally state that, other than the absence of prisoners at the objective, there were no major surprises in the operation."

Manor, who retired as a major general, firmly held to that position in a discussion a few days ago, although his action report also states that there was an intense firefight at the unintended second landing site. There the assault commander, Col. Arthur D. (Bull) Simons, and his assault "support group" were surprised to encounter "large orientals" -- who they were convinced were not North Vietnamese, and who were officially described this way: "These personnel were oriental, larger (5'10"-6') than other North Vietnamese Army [NVA] personnel in the area, and were not wearing the normal NVA dress but instead, wore T-shirts and fitted dark undershorts."

As a result of interviews with "Bull" Simons, who is now dead, and other members of the Special Forces assault team, writer Schemmer reported in a January 1980 article that the raiding party "killed 100-200 Chinese troops" in the unplanned portion of the action.

Manor and his deputy task force commander, then Brig. Gen. Donald D. Blackburn, both expressed great doubt last week that enemy casualties ran that high. There were 59 men in the entire assault force; Simons had 21 with him at the erroneous landing spot, where radio transmissions show that they spend only 2 minutes and 15 seconds in the actual firefight. Simons' group was extracted under fire by helicopter 8 1/2 minutes after it landed and then joined the main assault force at the prison compound, where about 50 enemy casualties were claimed -- North Vietnamese troops. Total combat casualties on the American side were listed as one: a sergeant "with a flesh wound in the inner thigh."

Gens. Manor and Blackburn, furthermore, at the time and since, regard the identity of the "large orientals" as "unknown" and "unproven." Chinese authorities in Washington said they knew of no PRC unit in North Vietnam that fits the description in the official American reports, and, in addition, Chinese military personnel do not wear either "T-shirts" or "fitted dark undershorts." So much for underwear analysis.

Schemmer, however, has reported that one of Simons' troopers stripped from a dead oriental a "brown leather field belt and raised star buckle" worn by Chinese officers. The belt is now reportedly in a display case but its owner declines to reveal his identity, because he never reported his find to his superiors.

As for Laird, his position is open: "These guys did such a hell of a job at Son Tay that is they thought they were Chinese, I'm prepared to go along with anything they say, even though I've got a question about it."

But perhaps most significant, from the standpoint of disclosing the extent of actual combat between Americans and Chinese during the Vietnam war, no serious official follow-up effort was made at intelligence levels to ascertain if the "large orientals" at Son Tay were really Chinese. That is "distinctly probable," the senior CIA action officer on the raid, George A. Carver Jr., then speical assistant to CIA director Richard N. Helms, said last week. But at the highest American levels, no one was anxious to find out if that was true. For if it was true, that would have been an awkward bit of information to have about a shooting engagement between Americans and Chinese that neither side had any interest in acknowledging.