Covering War: Rebels Reporting in Vietnam : ONCE UPON A DISTANT WAR, <i> By William Prochnau (Times Books: $27.50; 520 pp.)</i>

The opposite of the truth is a lie, by intention or by mistake, but the opposite of a lie is not necessarily the truth. This is why journalism, however brave and penetrating, needs a certain humility. Our crusades at best expose the lies of the powerful. Truth is out of our command; novelists and poets can get closer to it.

In the 1970s, Watergate brought down a hegemony of lies, but the teeth it grew on journalists altered a traditional balance of believability between government and the press, with negative as well as positive consequences. In the 1980s, the fall of a lying empire east of Berlin gave rise to a fistful of lying principalities.

William Prochnau’s “Once Upon a Distant War” tells of a remarkable two-year stretch in American journalism when one lie was overturned, only to give way to others. In the early 1960s a few reporters at what was then an out-of-the-way war insisted, in the face of pressure from all sides, on pointing out the failure of the United States’ growing but initially clandestine military and political involvement in Vietnam.

It was not true that the U.S. government, having adopted something called Project Beef-Up late in 1961, was restricting itself to providing a few hundred noncombatant military advisers and a little equipment. It was not just a lie, though, but a palpable lie, and witnesses were there to palp it.

An aircraft carrier with 40 helicopters strapped to its deck was hardly inconspicuous as it churned upriver through the flatlands outside Saigon. The Viet Cong could see it; so could the reporters. By the spring of 1962, U.S. military personnel were flying missions with the Vietnamese army, taking hits and casualties and, eager to talk about it, landing at their base a taxi ride from Saigon. The taxis arrived promptly: They carried Malcolm Browne of the AP, Neil Sheehan of UPI and Homer Bigart and David Halberstam of the New York Times.

By the end of 1963, with the overthrow and killing of President Ngo Dinh Diem and the collapse of the first stage of the U.S. effort, there were 78,000 Americans in Vietnam. The reporters boarded the copters, accompanied the patrols and noted the South Vietnamese troops’ lethargic incompetence against the Viet Cong and the despair of the American soldiers who went with them and began to die. They wrote about it.

Washington, which denounced Halberstam, Browne and Sheehan as bitter green youngsters (Bigart, a two-time Pulitzer Prize-winner and Korean War veteran, was presumably an embittered oldster), seemed to believe that foreign correspondents are foreigners who show up at State Department press briefings. The notion that 12,000 miles away they could be taking taxis, risking their lives, working hysterically, and despising the embassy and Army spokesmen who alternately shunned them and denied what they were seeing, eluded for a while the supposedly sophisticated policy-makers in the Kennedy Administration.

“Why don’t you get on the team?” was the celebrated outburst of Adm. Harry Felt, then the overall U.S. commander, to the journalists. They had. Only, as Prochnau’s book shows, it was a different team: not the generals and admirals and Washington policy-makers, but the majors and colonels who were in the field. They saw the failure of the policy of providing massive muscle to Ngo Dinh Diem’s corrupt commanders--who took the muscle and converted it to fat--and found that the journalists could be their means to say so.

“In effect the first coup in Vietnam was the coup of the American colonels over the American generals,” Prochnau writes. The coup would confront the Kennedy Administration with graphic on-the-spot accounts that contradicted the “can-do” reports it was both getting and encouraging (the Versailles syndrome) from its top military and diplomatic representatives.

The coup would lead to a switch in policy and the dispatch of a new ambassador, Henry Cabot Lodge, to supervise the overthrow of Diem. Reversing the Saigon embassy’s policy of hostility and stonewalling, Lodge, Prochnau writes, “would give the reporters everything they wanted: Attention. Respect. Even occasional information. Then he would give them Diem as well.”

The result would be, of course, a full-scale American involvement. Halberstam, Sheehan, Browne and others had played a significant and valiant role in destroying the lie that the old U.S. policy was working. This did not mean--and it was no fault of theirs--that the new policy would succeed or put an end to lying. Some of the gung-ho young officers who were the sources for the gung-ho young reporters went on to staunchly support the massive use of American land and air power. Sheehan would go on, after years of depression and painful work, to write a book, “A Bright and Shining Lie,” that casts a searching and ironic light on some of the truths that he and his colleagues thought they had found.

Prochnau’s book is a kind of avalanche. He moves a slew of material with a good deal of thundering. He can be portentous, emotionally lush and sometimes stagey. Beneath this, though, and despite a tendency to orchestrate two of its characters--Halberstam and Sheehan--with neo-Wagnerian clangor, he tells a complex story grippingly and well. Even better, beneath the dramatics, he displays a provocative intelligence.

In a sense, “A Distant War” is two books. One is about Vietnam and the military and political story that unfolded there between the U.S. decision in 1961 to step up its assistance and the coup that killed Diem two years later. Prochnau, who was in Vietnam years later as a Washington Post correspondent, narrates it with vivid and exciting detail. He writes, though, like a brilliant rewrite man whose editor is occasionally called away. Evoking the dizziness that Saigon could inflict on a new arrival, he tells us that “Vietnam was not simply exotic. It was erotic. And narcotic.”

Also: “In the cities women of exceptional Asian beauty--tiny, porcelain, ephemeral images of perfect grace--wafted past.” Also: “Down narrow Asian alleyways expatriate Chinese with three-strand beards and opaque eyes introduced visitors to the ancient pleasures of the poppy. . . .”

When Prochnau concentrates on the journalists, he turns from lavish rewrite man to sensitive reporter and thoughtful analyst. True, there are spells of lushness (Newsweek’s “devilishly suave Francois Sully”) that affect several of his otherwise memorable portraits. Dozens of pages share the existential and professional agonies of the very vocal Halberstam and Sheehan; though in the case of Halberstam, in particular, the author’s admiration is tempered by cool remarks about self-promotion.

I prefer his dryer figures: Browne, for instance, and Time’s Charles Mohr. They puzzle him more and he writes them better. There is the veteran Homer Bigart, who stammered, could never see the forest for the tree and, in Vietnam, was one of the first to notice that there was no tree. “All Homer has is a form of portable ignorance,” a colleague recalled. “He turns up knowing little; then finds out everything.”

Prochnau gives a masterly account of exploits, conflicts, difficulties and the sheer rush of journalistic adrenaline in covering a story that was part war, part intrigue, part pleasure--Saigon was deadly alluring in those early years--part danger and above all, the fact of doing battle every day with the bad guys. For the first time in an American war, the bad guys for these young bulls of the press often turned out to be what in other wars would have been the good guys: the general, the ambassador, the spook. Other enemies were the famous journalists who came out from Washington primed with the official line, and the young bulls’ nervous cowherds in the editors’ chairs back home.

Some of Prochnau’s best and most thoughtful material evokes the reporter’s fury at a distant editor’s restraints, and the editor’s worry about a distant reporter’s megalomania when not restrained. Prochnau, whose reporting of what went on at senior levels of UPI, Time and the New York Times is brilliantly vivid and balanced, and allows us to suspect that each side has a point.