That rare creature, a truly important movie, has just opened at the River Oaks Theater. I’m talking about the new documentary about Daniel Ellsberg, The Most Dangerous Man in America.

The film sketches in Ellsberg’s rather incredible life story, in which he goes from platoon-leading marine officer to cold war intellectual to anti-war saint, and at the same time illustrates how some of America’s most important institutions used to respond to challenges.

I don’t what made me want to cry more: the fury that you have to feel at the way five presidents, from Truman to Nixon, dug millions of American and Vietnamese graves with their lies. Or the grief at how far the media has fallen from the days when first the New York Times, then the Washington Post, and finally one newspaper after another, defied the Nixon Administration to print Ellsberg’s expose on the deliberate lies that had left out catastrophe in Southeast Asia.

But not only is Most Dangerous Man important, in that it illustrates history, but it’s also completely absorbing viewing. It’s also the most moving and exciting movie you could see this weekend.

Most readers will know that Ellsberg leaked the Pentagon Papers, the name given to the thousands of documents which successive administrations produced, with the somewhat mysterious intention of documenting their own lies. Many will know that he worked for, and purloined the Pentagon Papers from, the Rand Corporation, a think tank whose very name carries a whiff of Cold War banality and threat.

But I for one had no idea of how committed a hawk and cold warrior that Ellsberg had been before his conversion. In the mid-60s, when the former marine went to Vietnam to research the progress of the war, he carried a rifle and humped the terrain with the grunts. (Ellsberg refers to his days as a marine officer as the “happiest days of my professional life.”)

But he was also a thinking man, and an honest man. The movie, which was made by directors Judith Ehrlich and Rick Goldsmith, and narrated by Ellsberg himself, shows how, like many others, including Secretary of Defense Robert McNamara, he slowly became convinced that the war could not be won, largely because the Vietnamese were not going to allow themselves to be defeated. After a mission that resulted in an American route, Ellsberg remembers asking a solider if he “felt like a redcoat,” that is, like an unwelcome invader, and the solder replied, “I’ve been thinking that all day.”

But even though he saw that the war effort was doomed, Ellsberg wasn’t moved to try and stop it until he realized that the entire enterprise was based on a series of lies, starting with Truman’s secret support of France when that country attempted to recolonize Vietnam after World War II, during a time when the U.S. officially took an anti-colonial line.

Once he’d learned this basic fact (which he discovered by himself reading the so-called Pentagon Papers), Ellsberg began his historic--and perhaps immortal action—and pushed the New York Times and other into making history as well.

The film itself is truly gripping. The filmmakers unearthed some superbly illuminating footage, including scenes of Ellsberg on the ground in Vietnam. And, all these years later, Ellsberg is still quite a narrator. When he talks about sitting on a bathroom floor and crying “while my life split in two,” and the old cold warrior was burned away, he has you in the palm of his hand.

I have a few quibbles. The few recreations the directors include are pretty cheesy, considering the gravity of the story. And Ellsberg is always presented in hero mode. Even when he’s wrong, he’s wrong as a man-of-action, putting his own skin on the line. But—the man was a hero. Why not present him as one?

The film is disquieting when it exposes, for the 1000th time, the perfidy of Richard Nixon. You’ve probably read that he wanted to nuke North Vietnam, but when you hear Nixon actually say the words here, in one of his infamous tapes, it’s pretty chilling. As it is when he wonders how many Vietnamese would die if he obliterated the dikes on the Mekong River. “200,000? I don’t give a damn about the civilians.”

Those lines, spoken by the criminal himself, made me ashamed for having embraced, and forgiven, his character as portrayed by Frank Langella, so much in Frost/Nixon.

The Most Dangerous Man in America also makes me once again wish that we’d had an Ellsberg for our days, someone who could have told us what was really discussed during that secret confab Dick Cheney had with the oil company executives in spring 2001. Did the Iraqi oil fields come up in the conversation? Was there a map? It would be helpful to see it all in black and white.