It isn't any treat being dragged back to Vietnam, and to one of the ugliest decades in the history of this country, the 1960s; phrases like "search and destroy" and "peace with honor" and "credibility gap" ring bells some may feel are best left un-rung.

But "Vietnam: The Ten Thousand Day War," a 13-hour documentary series that premieres tonight on Channel 5, brings the Vietnam war into focus with enough clarity, authority and cool analytical insight to make this difficult journey rewarding and important. Everyone knows what those who cannot remember the past are condemned to do; of course, it's never been proven conclusively that remembering is really any great defense.

The documentary, conceived and produced by Canadian journalist Michael Maclear, and sternly narrated by Richard Basehart, was shown in Canada in 1980 as 26 half-hour episodes. For this American telecast, the programs have been edited into six two-hour blocks, plus a one-hour overview which airs tonight at 8, with the series continuing at 8 each night through Sunday. The series is an ambitious mix of combat footage, film of domestic reaction to the war, and new interviews with those who played key roles in the war-making, from Clark Clifford to Ellsworth Bunker to Nguyen Van Thieu to young men from Bardstown, Ky., who were among those who fought there and still don't quite know what they were fighting for.

Exhaustive, painstaking and often painful to watch, the series probes how and why America got into the debilitating mess and the complications that prolonged it. The dispassionate Canadian perspective--that of an interested and presumably unbiased outsider--seems worthwhile, although when anyone jabs at a scar this deep, they are bound to provoke some hostile reactions.

Maclear, the first Western TV journalist allowed into North Vietnam (in 1969), got access to North Vietnam's film archives for this report; there is a considerable amount of footage never seen here before. The fact that the Vietnamese point of view is taken into account may strike some die-hards as heretical, but perhaps enough time has passed for most people to view this material with something approaching objectivity.

Few of the principal characters seen in interviews throughout the series express anything resembling remorse, even though the program proceeds from the assumption that Vietnam was by every definition a disaster. Henry Cabot Lodge suggests that the whole tragedy can be traced to inadequate planning. Dean Rusk says that if President Kennedy had rushed in with 100,000 troops immediately, instead of starting small and escalating gradually, we might have won. Gen. William Westmoreland, intransigent to the end, growlingly blames "political decisions" for hampering the military. Melvin Laird declares, "I'm not sure America did lose in Vietnam."

And Alexander Haig contributes more of his own distinctive nonspeak, as when he says of Nixon's Cambodian bombing raids, "There were too many definitive limitations applied to the conduct of the announcement of the decision to go in there." The insight you get from guys like that making statements like that is thoroughly unintentional on their part, yet definitely worth the trouble.

Among those not interviewed are Robert MacNamara and Henry Kissinger.

Although the home front is covered throughout the series, the first half-hour of the fifth night (Friday) is devoted to "Front Line America," and to the violent antiwar movement that clearly became, for some, a mere excuse to celebrate radicalism. In an interview, Jerry Rubin talks about war protest of the time as the business of "creating an image" and says, "I saw the antiwar struggle as a moral crusade . . . If the Vietnam war did not exist, we would have had to create it." The preening egomania and perverse world view have not changed; Rubin talks of the torment of that period as if it were all a board game or a theatrical production. He seems as hateful a figure as Nixon. Perhaps they are the same person.

The less cynical, seemingly more genuine side of protest is also covered. David Harris, ex-husband of Joan Baez, notes that he served 20 months in jail for draft resistance--more than were served by any of the Watergate conspirators except for Hunt and Liddy. And the fervid, desperate temper of the times is brought back in news footage of a White House concert by the Ray Conniff Singers in which, before the group launches into "Ma, She's Makin' Eyes at Me," a young woman holds up a protest sign and beseeches Nixon to stop the bombing of Cambodia. "Bless the Berrigans and Daniel Ellsberg," she says.

Although the role of television in first prolonging, later curtailing this "first war fought without news censorship" is mentioned, it has been inadequately covered. One could quibble with other editorial decisions made, with the way the topics have been organized, and with the tactic of using combat footage as zippy punctuation and kinetic relief from the ruminations of all the talking heads.

But the program still looks to be valuable, perhaps cathartic, as a viewing experience and the final telecast, scheduled for Memorial Day, includes a haunting report on the plight of Vietnam veterans that is probably as good as last fall's Bill Moyers program on CBS.

"I still can't sleep without a light on," says a Marine who lost both legs in Vietnam and says his job there was to kill civilians. And Jack McCloskey, a Vietnam veteran who started a self-help program for fellow veterans traumatized by the war, says plaintively, "I want to regain my innocence." These 13 hours strongly suggest that this is something no one whose life was touched by Vietnam can ever do.