AMERICAN MAGAZINES and newspapers seem to have come to the decision that we are all ready to talk about Vietnam. Hollywood made box-office hay out of the war with "The Deer Hunter" and "Coming Home" and even had the gall to congratulate itself on Oscar night for admitting, years after the last American troops pulled out, that the war had actually occurred. Such brave souls, those spiritual leaders of Topanga Canyon.

But "Friendly Fire," the three-hour ABC movie version, at 8 tonight on Channel 7, of the C.D.B. Bryan book about an Iowa family's battle with the war and the war machine, does not have the opportunistic aroma of a big budget, Dolby stereo, blood-spurting extravaganza like "The Deer Hunter." Nor are there the tune-strewn, misty romantic delusions of "Coming Home." What "Friendly Fire" has is the integrity of reality; it is a true story about how a family's love of country was jolted by powers that were beyond their control and perhaps beyond any control at all.

In "Friendly Fire", we watch as, in essence, a Norman Rockwell painting is slashed to ribbons; because in trying to learn how their son, Michael, really died in Vietnam, Peg and Gene Mullen, of La Porte City, Iowa, not only became opponents of the war, but also unwilling pioneers of the new cynicism we have come to accept in the '70s as a way of life.

For the Mullens, and for the country, disillusionment with their own government was something new and disorienting. As we come to the end of a decade of demoralizing disclosures about the powers that be, and the powers that have been, "Friendly Fire" reminds us where it all began and of what happens when a "silent majority" remains silent.

There have been significant books about Vietnam-perhaps none more brilliant and troubling than Bryan's; there have been films, and there have been such shattering theatrical works as the David Rabe Trilogy, one play of which, "Sticks and Bones", was shown on CBS, after weeks of clumsy corporate hemming and hawing, in 1973-much closer to the war than "Friendly Fire" is now.

Any yet the national telecast of "Friendly Fire" may have a more devastating and purgative effect on the country than any of the previous works on the war, because watching it is like ending years of tearless, numbed mourning with one greaty cry. What it tells us is, we must not forget.

The direction of David Greene, the script by Fay Kanin, and the key performances of Carol Burnett and Ned Beatty as the bereaved and embittered Mullens, are all as conscientious and scrupulous as in anything ever done for television. But part of the impact of "Friendly Fire" may come as much from its timing as anything else; it is an act of emphatic punctuation for a nation still suffering post-Vietnam guilt and confusion.

There will be other dramatic and journalistic treatments of the Vietnam era, of course, but "Friendly Fire" could still be a landmark in the shaping of attitudes. It will be affecting on a national and personal scale to a degree that makes the big Hollywood movies look trifling. Television brought us this war, and now television is ending it.

Kanin, in Washington this week, said, "I think that right now people are ready to look at that war. I don't think it's that you give to them and make them ready.I think they signal you that they're ready. I just think it's time now, that people can take out all the pain and anger they felt about that war.

"It was the first war, really, that came to us on television, and I am glad that "Friendly Fire" was done for television. Mary Starger, the producer, had the opportunity to do it as a feature when he bought the book. He elected to do it for television for two reasons: one, because it was about a family and he felt that was a natural subject for television and, two, because, he said, the people of America lived with that war in their living rooms, and, dammit, we ought to talk about it now.

"Also," she added, "more people will see it if it's on television.

The Mullens we met in the book and see depicted in this film are not just an American Family; they are an ideal American family, iconographic in their agrarian simplicity. Director Greene makes everything we learn about them in the first minutes of the film seem resonant and symbolic-images of the land, the tractor that plows it, animals and trees and, indoors, mom at her sewing machine. In the room Michael shares with his younger brother John hangs a 4-H banner: "To make the best better."

But when he closes his closet door, the camera faces another image, the Army uniform that is hanging on it. The Mullens, who appear never to have questioned or dreamed of questioning the honor of their country or its leaders, are about to go through an incredible ordeal. Their son is not only taken from them, but they are persistently met with official indifference and bureaucratic obfuscation when they try to learn how and hwy he died.

The small lies they are told, about such things as the details of the body's escort back to the United States, come to represent the big lie of the war and the way it was rationalized to the American people. The final senselessness of their son's death-he was killed not in combat by by "friendly fire" from American troops, a casualty of human error-epitomizes the senselessness and waste of the war itself.

How the Mullens reacted and the way their anger grew reflect the country's growing dismay not only with the war but with the moral quality of its political leadership, and this was before Watergate battered whatever faith remained. When Peg Mullen's entreaties to Washington about her son's unexplained death result only in a form letter from the White House accompanied by the Xeroxed speeches of Richard Nixon, she writes on the envelope, "Return to Sender-Not Interested," and sends it back.

A schism between the governed and the government was only beginning. It has yet to heal.

When the Army offers the family the traditional gold star in memory of their son's death, Gene Mullen tells them, "Keep it." Yet they do not refuse the flag to be draped over his coffin, and they stand silently as "America the Beautiful" is sung while the casket is lowered into the grave.

Do the director and the screenwriter "lay it on too thick" in scenes like this? "Friendly Fire" is emotionally manipulative, but there is a clarity and finesse to Greene's style and Kanin's script that keep it from turning sanctimonious; the America the Beautiful" scene is restraint itself when compared to the "God Bless America" scene in the "The Deer Hunter."

Only in the second half of the film, when the journalist, Bryan, enters the story as a character (played by Sam Waterston) do the poignance and tension begin to let up; there is some narrative wandering, but in the last half hour, a part of the story is resolved when Burnett and Beatty leave the film for about 15 minutes and the scene shifts to Vietnam and a reconstruction of the night Michael died.

A greater part of the story, however, can never be resolved, and that is the whole point of telling it.

Dennis Erdman, who plays Michael, could not look more innocent of decent; he is virtually angelic, and the filmmakers could be faulted for making him perhaps too perfect. And yet boys like this did go to Vietnam, and thousands of them died there. Greene and Kanin and the actors make this family as believable as one's own; they are like characters from Thornton Wilderhs "Our Town" picked up and dropped into an age of insanity they cannot possibly fathom. To fathom it is to encourage it.

Details of the family's life are captured with a naturalism that makes them familiar and authentic, particularly the small moments-the nervous, awkward silences at the airport on the morning Michael is to leave for war, or the realistically disruptive slapstick of lose pigs running helter-skelter from their pens. Some scenes are accompanied by voiceovers of cold communications from the Pentagon or government spokesman; the juxtapositions are striking and not forced.

Burnett and Beatty are simply too fine for words. As their anger grows, the characters become inescapably heroic, yet never too large for life. "There will be a special hell for the men and government who allowed this war to continue," the mother writes in a letter to the president, and earlier, after guests at dinner have discreetly tried to change the subject from the war to the glories of Peg's turkey croquettes, she declares, "I don't want the subject changed." The determination in Burnett's eyes is eloquent.

Beatty, who more than proved his versatility to Washington audiences during his years at the Washington Theater Club and Arena Stage, and who has gone on to prove it again and again in film roles, is never at all patronizing toward the farmer he plays in "Friendly Fire", and when Greene crosscuts between Beatty doing chores at home and an approaching car that will bring him the news of his son's death, the sense of dread is unnerving, and the climax of the scene, thanks to Beatty, unforgettable.

How the family reacted to the media attention they received at the time, and their relationship toward Bryan, book contract in hand, are hardly as riveting as their hard-fought battle with the government, but these things are arguably an intrinsic part of the story, and they are dealt with intelligently.

Special progams like "Friendly Fire", and most TV shows made available for preview by networks and stations, do not include the commercials that will interrupt the programs when they are shown on the air. Some viewers react angrily when a program is recommended to them for its sensitivity and importance and is then repeatedly interrupted for shrieking, screaming pitches. There are only two possible solutions to this dilemma in a commercial television system: group the commercials tactfully, which the networks will not do, or make all television programs so frivolous that the interruptions won't matter.

Kanin thinks the commercial breaks will bot hurt "Friendly Fire" because they will give viewers a chance to compose themselves and prepare for the next trauma, indignity or challenge the Mullens will face. They are not depicted as perfect or unerring-their grief becomes obsessive, threatening to break what remains of the family apart-but their perseverance and courage, in the face of opposition from their own community, cannot be seen as anything but brave, nor "Friendly Fire" as anything but extraordinary. The essential message of prime time programming usually is, "Forget." This is a program that says, "Remember." CAPTION: Picture, Carol Burnett and Ned Beatty as the embittered Mullens-Unforgetable.