If history is any guide, the daily briefings from Defense Secretary Donald H. Rumsfeld could prove an imperfect portrait of the unfolding war in Afghanistan.

No one has accused Rumsfeld of misleading the press or misrepresenting events on the ground. But post-mortems on previous American wars routinely found that the government exaggerated its successes and minimized its setbacks in its public presentations; indeed the tendency to embroider has been documented back to the Civil War.

At moments of national crisis, officials in Washington have on occasion made statements they knew were flat-out untrue -- the way President Eisenhower did in describing the U-2 spy plane shot down over the Soviet Union in 1960 as a weather research plane. President Nixon hid a massive bombing campaign in Cambodia for more than a year, even falsifying reports sent to the Senate.

More often, officials in wartime have given the press and public a partial picture meant to portray events in the best possible light, analysts say. Over time, that instinct has produced inflated accounts of enemy soldiers killed in Vietnam, Iraqi Scud missiles intercepted in the Persian Gulf War and Serbian tanks destroyed in Kosovo.

"Most of the time, the problem in briefings is not that there is some blatant mistruth being uttered," says Tom Rosenstiel, director of the nonpartisan Project for Excellence in Journalism. "It's that what you are getting is a very selective thing that is designed to accentuate the positive."

This tendency to highlight good news can reflect legitimate disagreements about how to interpret ambiguous intelligence information on the tight deadlines that daily briefings impose, experts say. It may spring from a reluctance to release any information that could prove comforting to the enemy -- a factor that could have been a motivation in the inflated accounts of the Patriot missile's success in destroying Scud warheads during the Gulf War.

But critics warn this dynamic can also reflect the urge to only report results that maximize public support for the war effort -- particularly in cases like this, where few reporters can challenge the Pentagon's portrayal through independent access to the battlefield.

"Flat-out lies have occurred but are probably less the problem. Far more numerous are cases of deception, dishonesty and withholding of information," says Michael Sherry, a historian at Northwestern University.

So far, most observers say Rumsfeld has been circumspect in his claims of success; in one briefing this week, he acknowledged "the cruise missiles and bombers are not going to solve this problem." Rumsfeld has also pledged not to lie to the public, although he has refused, on some occasions, to answer questions he considered inappropriate.

Throughout history, truth has been a scarce enough commodity during military conflict that Sen. Hiram Johnson famously described it as "the first casualty" of wartime. For almost as long as Americans have gone to war, the military and the press have engaged in their own hostilities over the quantity, and accuracy, of information provided.

As far back as the Civil War, Secretary of War Edwin M. Stanton personally altered casualty figures before releasing them, reducing the number of men General Ulysses S. Grant lost in one engagement by two-thirds, reported Phillip Knightley in his book "The First Casualty," a classic history of war reporting.

Gen. Douglas MacArthur, the flamboyant commander of American forces in Southwest Asia during World War II, was a particular offender. "His news releases bore so little resemblance to what was actually happening that they aroused resentment among the troops," Knightley wrote.

But veracity was enough of a problem in Vietnam that the notion of a "credibility gap" between the government and the public was first born in the military's daily Saigon briefings.

Over time, the daily briefings became known as the "5 o'clock follies" -- a label that underscores how little stock reporters put in their accuracy. To a considerable extent, the briefings "were bedeviled by the problem" of precisely describing the state of "an unconventional war," wrote Peter Braestrup, the former Washington Post Saigon bureau chief in "Big Story," his study of the media during Vietnam.

But most reporters felt the broader problem was that the military, under pressure from political leaders in Washington to show progress, systematically exaggerated their gains, particularly in the daily "body count" -- the number of enemy soldiers killed. With reporters freer than in past -- or future -- wars to explore the battlefield themselves, the discrepancy between what they saw -- and what they heard from the podium -- opened a credibility gap that helped erode support for the war at home.

The televised daily briefings now conducted by Rumsfeld and Gen. Richard B. Myers, the chairman of the Joint Chiefs of Staff, represent the Pentagon's best thinking about how to close the credibility gap that opened in Vietnam. The tradition of using top officials to brief on a war's progress began during the Gulf War, when 24-hour cable television made it possible for senior commanders like Gen. H. Norman Schwarzkopf in Saudi Arabia, and then-Defense Secretary Dick Cheney in Washington to reach the public directly. As Cheney explained after the war, the daily briefings allowed him "to manage the information flow" without the "filter" of the press.

Coupled with severe restrictions on media access to the battlefield, that model proved so successful at solidifying public support for the Gulf War that it was followed by the Clinton administration in Kosovo, and so far by the Bush administration in Afghanistan. In each instance, the administration made the highest officials available as the principal source of information -- in effect, making them the face of war.

Yet centralizing briefings at the top hasn't eliminated questions about credibility, especially after the war ends.

After the Gulf War, several studies challenged the image of precision, high-tech bombing that dominated both the official briefings and television coverage of the war. Contrary to dramatic footage at one briefing that apparently showed American bombs destroying a mobile launcher carrying Iraqi Scud missiles, the official Air Force review of the war found no evidence that any mobile Scuds were destroyed from the air.

The second look was even less flattering to the Patriot missile system that the U.S. deployed to shoot down the Iraqi Scuds. In February 1991, then-President Bush claimed the Patriots had downed 41 of 42 Scud missiles fired during the war.

But an exhaustive study by Theodore Postol, a professor of science, technology and national security policy at the Massachusetts Institute of Technology, later found no evidence that a Patriot had disabled the warhead on any Scud missile. "There were three cases where damage was done to the airframe of Scuds, but in all three cases the warheads went on undamaged," said Postol. "So basically there are no cases that we studied -- and we had three-quarters of the engagements on film -- where we saw evidence of a destruction of the Scud warhead."

Later, William S. Cohen, the Defense secretary under President Clinton, acknowledged that the Gulf War-era Patriot missile "didn't work."

Critics also raised questions about whether the military overstated the accuracy of the formidable aerial bombardment and cruise missile attacks during the war. "In fact things were going well," said Postol. "They just weren't going as astonishingly well as they were claiming."

Cohen and the Clinton administration faced their own questions after the 1999 NATO intervention in Kosovo. Last year, Newsweek obtained a classified Air Force study that concluded the Pentagon and NATO, in their public briefings, had grossly exaggerated the number of Serbian tanks and artillery pieces destroyed in the bombing. The Clinton administration also got into trouble by reflexively denying Serbian claims of civilian casualties, some of which proved accurate, recalled former Clinton Press Secretary Joe Lockhart.

Why does the official version of progress on the battlefield so often need revision? Lockhart said the problem isn't so much malign intent as pressure that daily briefings and news deadlines create for unrealistic clarity amid the ambiguity of war. "I don't think it's the government in a conspiratorial way deciding to deceive people," he said. "It's human beings who will jump on positive information quicker than they will on negative or neutral information."

In other cases, the military may withhold information that could endanger operations or provide useful intelligence to the enemy.

But the explanation for inaccuracies in briefings isn't always so benign, says Jacqueline Sharkey, a professor of journalism at the University of Arizona and author of a recent book on relations between the military and the press.

The perennial problem, she says, is when briefings go "beyond trying to control information that would affect operational security and troop safety and [try] to control political perceptions and public opinion about the decision to go to war."

Los Angeles Times staff writer Marisa Schultz contributed to this report.