(Link to full text, below...those of you with more knowledge than me will please comment...)
Since the late 1940's, the conventional wisdom had been that the U.S. drifted into the war in Vietnam following the French colonial period, and that American Presidents were given bad information by their advisors and thus made mistakes in policy that led to deeper and deeper involvement. (Daniel) Ellsberg discovered that this view was incorrect.
Contrary to this version of events pushed by the government, the U.S. didn't "drift" into anything. The closest advisers to five presidents (Truman, Eisenhower, Kennedy, Johnson and Nixon) told their bosses the truth from the outset, that there was no way the U.S. could win a victory against post-colonialist Vietnamese nationalism; the best that could be hoped for was endless stalemate. Despite the warnings, those Presidents not only embraced the war but kept expanding it. Millions died as a result.
The key to carrying on that insane, immoral war was that the decisions always were made in secret by the President, away from scrutiny by the Congress, the press, and certainly by the American people.
In other words, both Republican and Democratic Presidents and their closest advisers lied for decades to the citizenry, to the press, to the Congress -- the result of which was untold misery for both U.S. military troops and Vietnamese civilians.
The common wisdom is that "you can't keep secrets in Washington," and that someone always deliberately leaks or inadvertently blabs. But, says Ellsberg, who was privy to some of the most top-secret material for years, "the fact is that the overwhelming majority of secrets do not leak to the American public. This is true even when the information withheld is well known to an enemy and when it is clearly essential to the functioning of the congressional war power and to any democratic control of foreign policy...Secrets that would be of the greatest importance to many of them can be kept from them reliably for decades by the Executive Branch, even though they are known to thousands of insiders."
Daniel Ellsberg's "Secrets"