The Weather Underground -- the most violent splinter group spawned by the late 1960s political counterculture -- was once a vivid symbol of the romance, danger, and self-delusion of the radical left. Sam Green and Bill Siegel's provocative documentary puts you in the eye of that vanished era: If the idealism that leads to terrorist acts now largely seems an offshore phenomenon, their film is a reminder -- and for those who never knew, a bracing lesson -- that it can and did happen here.

Using sharply edited historical footage, "The Weather Underground" re-creates the pressure cooker of late-'60s America: the battle for civil rights in the South, the escalating war in Vietnam and mounting resistance at home, the sickening string of assassinations. The era was a bloody carnival that seemed like the end of the world, and to the youth culture then erupting, building a new world seemed the only possible response. If one couldn't be created by peaceful methods, then force would happily be applied.

This is where the Weathermen came in. Taking their name from a Dylan lyric ("You don't need a weatherman to know which way the wind blows"), the group hijacked the more pacifist Students for a Democratic Society at their national convention in 1969. With such charismatic leaders as Rudd, Bernardine Dohrn, and Bill Ayers, they came off as articulate, committed, and chic, and they called for the immediate overthrow of the government.

They had the explosives to back it up. After the announced "Days of Rage" action brought out only 500 people, the group redubbed themselves the Weather Underground, dropped from public sight, and set about bombing selected targets. In the end they were fatal to themselves -- three of the members died in April 1970 when a bomb exploded prematurely in a Greenwich Village townhouse, but it's worth noting that that device had been intended for an army officers' dance in New Jersey. If zealotry doesn't always lead to murder, it's not for lack of trying.

Green and Siegel are sympathetic to the group's ideals and properly critical of the results. They show us the members of the Underground in their antiheroic youth, often in moments that are emblematic of their passion and callowness. Here's the famous shot of Weatherman John Jacobs at a Days of Rage rally in a football helmet, turning the iconography of Jock America inside out. Here's Dohrn declaiming to the press in 1969 that "white youths have to choose sides now," looking every bit the glamorous revolutionary.

And here they are now, in interview segments in which they're graying and caught in various stages of regret. Dohrn and her husband, Ayers, have come in from the cold -- the charges against many of the Weathermen were dropped because of the FBI's heavy-handed tactics -- but they remain politically active and, it is clear, in a certain state of denial.

David Gilbert nurses his bad conscience in prison after a 1980s Brinks truck robbery that turned fatal. Brian Flanagan, from his Manhattan bar, quietly notes that the moral high ground "is a dangerous position to take."

That they are now largely forgotten is testimony to the way history swallows idealism, digests it, and moves on. "The Weather Underground" is essential viewing for anyone who wants to know the roots -- and perils -- of modern political dissent.