Tactic: Use a fraction of reality

``DISINFORMATION is most effective in a very narrow context,'' says Frank Snepp in an interview. ``It's most effective when it pertains to something the press has no access to, or information which is exclusively in the intelligence community: radio intercepts, spy photos.''

Mr. Snepp is a disillusioned former CIA agent who honed his expertise in disinformation while briefing reporters in Vietnam. He became a center of controversy in the United States when he published -- without CIA clearance -- a book about the fall of Saigon.

``You take a fraction of reality and expand on it. It's very seldom totally at odds with the facts,'' Snepp says of one approach to disinformation.

``We were trying to suggest to Congress in 1974 that more aid was necessary because the Communist threat was increasing, so we talked about infiltration of Communist forces to the south and led everyone to believe they had been expanded by 60,000. But we neglected to tell them 60,000 had been killed, captured, or dispatched back. It's shaving a piece of reality off.''

Snepp continues: ``Disinformation in the CIA sense is not false information. That is the grossest kind, and that is the kind you can usually be caught out on. When the CIA does it, it's nothing so gross. It's information which keys off of reality, like docu-drama. But that's the CIA definition, which is not to take an untruth, but to take a piece of truth.''

Asked for an example from 1973, he describes feeding a story to the Economist magazine ``to create the impression that the Communists were trying to build a third Vietnam on the western border of South Vietnam, where they could set up airfields, antiaircraft, a fortified separate Communist entity.'' The object was to convince Congress that the cease-fire would not hold, he says.

``What we did was to take very scattered, questionable intelligence, intelligence that seemed to fit our theory, pieced it together, and made a mosaic, not indicating a lot of countervailing evidence. Though there were plans for a road, for example, there was no evidence that they were really building it; they were just contemplating this. . . . That was disinformation. It wasn't a lie.''