The 21-year-old Californian, who says his spiritual journey began when he first saw Spike Lee's ''Malcolm X'' when he was 12, told a packed courtroom here, ''I made a mistake by joining the Taliban.''

''I want the court to know, and I want the American people to know, that had I realized then what I know now about the Taliban, I would never have joined them,'' he said. [Excerpts, Page A10.]

While he expressed ''remorse for what's happened,'' it was not clear to what he was referring. His only explicit expression of regret was to his family. ''I know they have experienced a tremendous amount of pain through this past year, and for that I am sorry,'' he said.

Mr. Lindh's mother, father, older brother and younger sister were sitting behind him in court as he faltered his way through his statement. He was crying outright when he described ''the atrocities committed by the Northern Alliance against civilians,'' adding: ''I went to Afghanistan because I believed there was no way to alleviate the suffering of the Afghan people aside from military action. I did not go to fight against America, and I never did.''

His father, Frank Lindh, said later that his son's statement ''was really painful to hear, but it was beautiful.''

Though it was Mr. Lindh's fate that was being sealed today, he was only one of three players in an emotional two-hour courtroom drama that concluded the nation's first legal case in the post-Sept. 11 campaign against terrorism.

The second figure was Johnny Spann, whose son, Johnny Micheal Spann was the C.I.A. officer killed during the uprising at the prison in Mazar-i-Sharif, Afghanistan, where Mr. Lindh was being held last November.

In 18 sorrowful and angry minutes standing before the judge, Mr. Spann, a Realtor in Winfield, Ala., all but accused Mr. Lindh of murdering his son. He was incredulous at Mr. Lindh's statements and objected strenuously to a prison term that he deemed too lenient.

Mr. Spann's remarks elicited a clear-eyed response from the third player -- Judge T. S. Ellis III of Federal District Court.

Judge Ellis made it plain that the government had not produced even a scrap of evidence to suggest that Mr. Lindh had anything to do with the death of Mr. Spann's son.

''Your suspicions aren't enough to warrant a jury conviction,'' Judge Ellis told the father.

Mr. Spann spoke of the medical examiner's report and the trajectory of the fatal bullet, suggesting that his son had been shoved to his knees and executed at close range in the exact location where Mr. Lindh was reported to be.

''Proximity is not guilt,'' the judge said. He told the father that his son, a former marine, was clearly a hero, but added, with a directness that seemed to surprise Mr. Spann, ''Of all the things he fought for, one of them is that we don't convict people in the absence of proof beyond a reasonable doubt.''

Mr. Lindh's sentence was proposed by the prosecution and defense in a plea agreement announced in July. Of 10 counts originally brought against him, he pleaded guilty to charges of aiding the Taliban and carrying explosives. He could get time off for good behavior, but he has to serve at least 17 years.

The deal spared Mr. Lindh life in prison and required him to continue his cooperation with the government.

Investigators' accounts of their interviews with Mr. Lindh, contained in documents made available to some news organizations including The New York Times, say that Mr. Lindh told them that Osama bin Laden's original plan was to attack the United States in three phases.

The documents, which the government has not corroborated, said the first phase consisted of five attacks, though they listed only four: two on the World Trade Center, one on the Pentagon and one on White House.

The second phase was to occur immediately before Ramadan last year. No location was identified but it was to involve unspecified biological agents, possible attacks on natural gas resources or nuclear weapons facilities in the United States. It was supposed to be so big that it would ''make the United States forget about the first phase,'' according to the documents.

The third phase was supposed to ''finish the United States'' and take place within six months of the Sept. 11 attacks, the documents stated.

Mr. Lindh also provided considerable detail about his experience in an Al Qaeda training camp. According to the documents, he told investigators, ''Students learn the physics of explosions and how the use of explosives can put pressure on the government by instilling fear in the people.''

Mr. Lindh also described the training camp in Afghanistan, situated approximately two hours ''by car, west of Kandahar,'' where, he said, ''a majority of the trainees are from Saudi Arabia.''

According to the documents, the camp's director was a man named Emir Abdul Qaddoos, who Mr. Lindh said was ''an Iraqi who passes as an Afghani.''

After hearing from all sides, Judge Ellis asked Mr. Lindh, who was wearing thick black glasses and a drab olive prison jumpsuit, to stand before him.

''Life is about making choices and living with the consequences,'' the judge said. ''You made a bad choice to join the Taliban and engage in that effort over there.''

He noted that Mr. Lindh had not said that he had no role in Mr. Spann's death, at which point Mr. Lindh said in a strong voice, ''That's correct, sir; I had no role in the death of Johnny Mike Spann.''

The judge sounded puzzled and annoyed by one thing, noting that Mr. Lindh had told his interrogators that once he realized he had ideological differences with Al Qaeda, it was too late for him to leave because he was afraid he would be killed. He told Mr. Lindh to reflect on why he did nothing, despite knowing that Al Qaeda was planning attacks on the United States.

''You were willing to give your life for the Taliban but not for your country,'' the judge said. ''Fighting for something you believe in is a virtue, but only if the belief is virtuous.''

He took note that Mr. Lindh was bright and still wanted to study many other cultures, and concluded with this: ''I hope you will also study your country and the protections that were accorded to you.''

He said he hoped Mr. Lindh would have ''as much fervor'' for the American system of justice ''as you had for some causes you believed in in the past.''

Photo: Leaving the federal courthouse in Alexandria, Va., after John Walker Lindh's sentencing was his team of lawyers, led by James Brosnahan, center. (Paul Hosefros/The New York Times)(pg. A10)