Revealing new evidence of official American wrongdoing during the Vietnam war may seem of only passing historical importance given all the water that has flowed under the bridge since that unhappy conflict. But tonight's CBS Reports "The Uncounted Enemy: A Vietnam Deception" (Channel 9, 9:30 p.m.) is both powerful and timely.

It is powerful because it is so thorough. A series of retired senior officers acknowledge on camera that figures of enemy strength were systematically underestimated. Why? As the former U.S. commander in Vietnam, Gen. William C. Westmoreland, abashedly acknowledges, "The people in Washington were not sophisticated enough to understand and evaluate this thing and neither was the media."

In other words, to show that we were winning the war, the number of enemy forces was cooked.

The program is timely because a revisionist strain in connection with Vietnam has appeared. This view, propounded chiefly by former correspondents Peter Braestrup and Robert Elegant, holds that an anti-government journalistic bias contributed to a defeatist attitude on the home front when the war was actually being won. In particular, Braestrup contends that the 1968 Tet offensive was misreported as a Viet Cong victory when actually it was a disastrous setback.

"The Uncounted Enemy" is a persuasive reminder that whatever significance negative reporting may have had on American attitudes, repeated instances of official duplicity also led to popular cynicism and antagonism toward the war. Because U.S. leaders in Washington and Saigon felt a compulsion to show progress in the fighting, they dissembled. Instead of a triumph over the Viet Cong in early 1968, as Westmoreland and others were promising, the nation witnessed an all-out enemy assault.

Now we learn from CBS that the commander knew enemy strength was substantially greater than Washington and the public were being told, but withheld the information. That is serious business and Westmoreland is predictably chagrined at its disclosure now. He angrily denounced the program yesterday in an interview with the Charleston (S.C.) News and Courier, terming it "vicious" and "irresponsible journalism."

But what television has over print journalism in a situation like this is the uncontestability of taped interviews. There can be no insistence that sources were misquoted when their words and faces are on the screen for all to see. Retired Col. Gaines Hawkins of Army Intelligence, a grizzled old veteran, tells reporter-producer George Crile of enemy numbers that the U.S. command concocted: "These figures were crap."

In all, at least six military officers are interviewed on camera by Crile and CBS correspondent Mike Wallace about their part in the deception. Their accounts are gripping and their testimony is at times so obviously confessional that it is painful to watch. Former Col. George Hamscher of the Defense Intelligence Agency admits the numbers: "It was a lousy strength estimation. It was shoddy. But we did it."

Westmoreland appears at intervals throughout the program in a classic Wallace interview. At the outset the general seems calm and confident. As the story unfolds his manner dissolves into twitches and stuttering. There is nothing especially uplifting about Westmoreland's discomfort but it, too, adds to the program's credibility.

Official dishonesty surrounding Vietnam comes as no surprise (in fact, allegations of underestimated enemy strength were first made a decade ago). Yet, what CBS Reports presents is an outrageous abuse. Americans were asked to support an effort in Vietnam that may well have had a legitimate purpose. But then those same Americans were lied to. This is a first-rate program.