If "Who'll Stop the Rain" isn't a knockout, destined to take its place among such rarefied, thought-provoking adventure classics as "the Treasure of Sierra Madre" and "The Wages of Fear," I haven't been movie-crazy all my life.

The movie is a stunning example of collaborative fidelity and artistry directed by Karel Reisz, and its impact may be heightened if one is in the dark as to the plot of its literary source, Robert Stone's "Dog Soldiers." Suddenly you find yourself in the grip of an overwhelming cinemate and melodramatic undertow, at once thrilled, astonished and dreadfully uncertain of where it may set you down.

It doesn't set you down gently. The circumstances are too sinister and perilous and the implications too sobering to permit either the audience or the misguided but essentially decent characters to come out unscathed. When John Converse, a correspondent nearing the end of a debilitating, terrifying tour in Vietnam, rationalizes a drug smuggling scheme in which his wife, Marge, and a resourceful old friend named Ray Hicks are also implicated, he buys a harrowing quantity of trouble.

Unable to foresee the consequences of this feckless lapse into wartime profiteering. Converse becomes a pawn in the hands of confirmed, ruthless criminals, a chillingly sardonic trio dominated by a crooked narc named Antheil and conceived in the tradition of the title characters in Hemingway's story "The Killers." Ultimately, the survival of the Converses hinges on the innate heroism of Hicks, a romantic soldier of fortune denied a conventional but curiously ennobled by his stubborn intergrity.

Nick Nolte, unaccountably coarse and dislikable as an upper middle-class adventurer in "The Deep." comes roaring back like a champion in the fabulous role of Ray Hicks. Nolte may have been born to embody diamonds-in-the-rough. He is in exceptionally fast acting company in "Who'll Stop the Rain," a factor that makes his dominance all the more impressive. Sparkling performances surround him, yet Nolte emerges clearly as the most powerful and affecting presence on the screen.

With his blond hair combed straight back Nolte bears a striking resemblance to Dany Duryea, but when was Duryea ever this rugged, explosive and stirring? Nolte's sheer physically becomes more potent by virtue of his expressive control, which one presumes that Karel Reisz helped to encourage and refine.

Contemporary moviegoers may feel as wowed by Nolte in this role as their counterparts were by Brando as Stanley Kowalski a generation ago. He makes Hicks an impressive and even mean-looking stud. You would not want to tangle with this guy. At the same time one can perceive oddly affecting depths and vulnerabilities lurking behind the guarded, narrow-eyed stud's facade.

A nobler brute than Kowalski, Hicks is the latest great addition to the tradition of the Good Desperado, redeemed from a disreputable or wasted life when the chips are down by his willingness to defend the weak and risk all for love or friendship. It doesn't matter that the Converses seem unworthy objects of Hicks' affection in some respects. It certainly won't prevent him from achieving a cinematic immortality comparable to that of Shane or Pepe Le Moko.

When Nolte arms for the showdown with Antheil and his thugs, you can't help anticipating a brave new chapter in action movie history. You aren't let down. Moreover, Reisz sustains the aftermath longer than one would have thought possible, enabling Nolte to make not one but three brilliant "exit" scenes.

A remarkably faithful and astute adaptation of a strong novel, the film condenses Stone's book, which won the National Book Award for fiction in 1975, without doing irreparable damage to its story, principal characters or larger meanings. The screenplay, completed by Judith Rascoe after Stone himself had worked on two earlier drafts with Reisz, strips the book for action and suspense without sacrificing philosophical and polemical dimensions. The filmmakers don't stop for digressions, as the novelist did. They succeed in expressing his meanings implicitly.

"Who'll Stop the Rain" may be appreciated as a loaded chase melodrama, a parable about the corrosive influence of the Vietnam war on American morale, mores and mythologies. The book's sense of trying to expiate some of the guilt feelings troubling people in sympathy with the anti-war movement is enhanced in the film. It's difficult to escape the conclusion that the Ray Hickses of the world - the brave and capable common men - pay inordinately heavy dues for the follies of the John Converses - the weak, mixed-up, sleep-walking "intellectuals" and "commentators."

Converse justifies his inept role as a heroin smuggler after he is nearly killed in a misdirected bombing mission. Citing an earlier bombing atrocity in which elephants were targeted on the assumption that they might be supply animals for they might be supply animals for the Viet Cong, Converse remarks, "If the world is going to contain elephants pursued by flying men, people are just naturally going to want to get high."

He flatters himself that he's finally making a resolute gesture and that a small killing in dope - a promised $25,000 - will repay him for the sight of needless carnage and the intimate knowledge of his own fear of death. As a result, he precipitates a sequence of events that turn Marge, an unwitting accomplice in the movie adaptation of the story, and Hicks, a former Marine buddy now in the merchant marine who agrees to hide the cache on his ship and deliver it to Marge in Berkeley, into fugitives. Returning soon after they take it on the lam. Converse is quickly picked up and roughed up by Antheil's hirelings.

Stone himself makes it rough on Converse. Indeed, he takes a perversely keen pleasure in ridiculing writers for their vanities and weaknesses. Confronted by his seething father-in-law, a Berkeley bookstore proprietor, Converse feebly explains his criminal escapade as follows: "I've been waiting all my life to - up like this."

Perhaps no other actor in the country is better equipped, physically and temperamentally, to make such a humiliating confession sound authentically funny than Michael Moriarty. His potential genius for embodying weaklings is at last being exploited intelligently. Moriarty enables us to believe in Converse's inadequacy without encouraging us to hate him for it. He needs to retain at least a scrap of self-respect in order to find his courage at a climactic moment in the story, and Moriarty rationalizes this neo-Conradian conception. He's a coward, but not a craven, shameless coward. He's closer to most of us than we care to acknowledge.

As the father-in-law, David Opatashu is entrusted with a put-down that seems devastating both in context and in retrospect: "A sense of unreality is not a legal defense." Converse certainly has no viable legal or moral defense for his stupidity, but his sense of unreality is real enough. Reisz and cinematographer Richard Kline help to clarify it by endowing the action with a slightly hallucinatory vividness.

Kline photographed Brian De Palma's "The Fury," and he has a way of alternating and blending florid, burnished colors with shadowy, ominous colors that creates a pervasive mood of apprehension. There are no sanctuaries in these settings, which range from war zone to contrasting apartments in Saigon, Berkeley and Beverly Hills to a decaying ruin of the counterculture in the mountains of New Mexico. The atmosphere is consistently unsettling.

Tuesday Weld may be considered the symbol of renewal in this fable of survival and redemption in a time of demoralization. She returns from almost a decade of limited or non-existent opportunities, to create a touching impression with Marge. The character is softer than it was in the book - not so far gone down the road to either loneliness or addiction. The movie's approach salvages some compassion for the Converses, and one seems justified in hoping that they can pick up the pieces.

Richard Masur, the cuddly suitor of "One Day at a Time," is a threatening revelation in the role of Danskin, the wisecracking half of a strong-arm team whose stoogey component is embodied with equal wit by Ray Sharkey, whom a few confirmed movie freaks may recognize as the nice-guy lead of Martin Brest's "Hot Tomorrows." All the villains are compulsive jokers and insult-artists who become comic in another respect: Their greed and ruthlessness reach such extremes that they're transformed into comic monsters, like the conspirators in "The Maltese Falcon" or the bandits in "Sierra Madre."

Anthony Zerbe and Charles Haid bring impeccably snide self-confidence to the roles of the big operators. Antheil and Eddie Peace, respectively. These hateful smarties have authority. You can see where their ability to get the numbers of less calculating or corrupt people redounds to their advantage. You can also see how it leads them to overrate themselves and make crucial miscalculations of their own.

"Who'll Stop the Rain" illustrates what can happen when Hollywood doesn't blow it, when all the ingredients for a classic popular movie come together. Last year "Looking for Mr. Goodbar" illustrated what happens when everything falls apart. The film-makers responsible for "Who'll Stop the Rain" evidently preserved their nerve and sense of purpose. One can't say the same of the distributor, United Artists, which belatedly changed the title from "Dog Soldiers" and seems to be launching the film as a summer afterthought.

"Who'll Stop the Rain" is far and away the best new movie in 1978.