The agreement says that for the rest of his life the government may immediately and unilaterally capture and detain Mr. Lindh as an ''enemy combatant'' should it determine that he has engaged in any of a score of crimes of terrorism. The government has said that such detentions, which are military rather than criminal, are beyond the power of the courts to second-guess.

Legal experts said the reference to enemy combatant status in the plea agreement, along with the government's recent decisions to detain Yasser Esam Hamdi and Jose Padilla as combatants without filing charges against them, suggests that the government now prefers detentions to trials. Officials accuse Mr. Padilla of planning to explode a radioactive device, while Mr. Hamdi was captured on the battlefield in Afghanistan. Both men claim American citizenship.

Some experts read the outcome in the Lindh case differently, saying it provides proof that the courts remain well suited to hear terrorism cases. After all, the government obtained a substantial sentence apparently without having to disclose sensitive information or otherwise compromise its fight against on terrorism.

Douglas Cassel, the director of the Center for International Human Rights in Chicago, said the 20-year sentence Mr. Lindh accepted, which is subject to the judge's approval, sends a strong message.

''Even if you are a very young man and even if you don't directly engage in acts of violence against Americans, you can get a very significant sentence,'' Professor Cassel said.

By comparison, Ronald Allen, who teaches criminal law at Northwestern University, said only the gravest crimes warranted a sentence that long against a first offender. ''You would probably have to kill more than one person,'' Professor Allen said.

Professor Cassel said the severity of the sentence meant that the government should not hesitate to file criminal charges against Mr. Hamdi and Mr. Padilla.

''The first and most basic thing is they ought to very carefully review those cases,'' he said.

But Eric M. Freedman, a law professor at Hofstra University, said that he thought the government did less well in the Lindh negotiations and that he was worried that it would try to avoid the courts from now on.

''The United States government has suffered a setback in this, which is good for Lindh but bad for civil liberties, especially if the lesson the government learned is that it's easier to do these things without due process than with due process,'' Professor Freedman said.

Yale Kamisar, a law professor at the University of Michigan, said the procedure and outcome in the Lindh case should be considered satisfactory to the American notion of justice, especially in contrast to the alternative of indefinite detention.

''At least in this case he got his day in court,'' Professor Kamisar said of Mr. Lindh.

''It may be that if this case arose today they wouldn't do it this way,'' he said, referring to the government's choice of federal court for its forum. ''They've toughened up.''

Legal scholars found it hard to identify a rationale that would call for an ordinary criminal prosecution of Mr. Lindh but military detention of Mr. Padilla and Mr. Hamdi. The search for a unifying principle becomes even more difficult if Zacarias Moussaoui and Richard C. Reid are added to the mix. Mr. Moussaoui is charged in connection with the Sept. 11 attacks, and Mr. Reid is accused of trying to detonate an explosive device in his shoe on an airplane. Cases against both men are proceeding in federal court.

Efforts to distinguish the treatment of these prisoners on consistent grounds tend to fail. The distinguishing factor is not citizenship: Mr. Moussaoui is French, and Mr. Reid is British; the others claim American citizenship. Nor is it the place of arrest: Mr. Lindh and Mr. Hamdi were captured in Afghanistan, the others in the United States. Nor is it the nature of the central criminal charge: Mr. Moussaoui, Mr. Reid and Mr. Padilla are accused of attempting or conspiring to commit terrorist acts, the others of fighting on the wrong side abroad.

''You do worry about equal treatment and having a consistent theory about who ends up where,'' said Ruth Wedgwood, a law professor at Yale.

The only factor that seems to explain the disparity in how the men were treated is time. The later detentions were military, suggesting that the government may now view ordinary trials as more trouble than they are worth.

''Some people may be feeling a little regret that Moussaoui is using the courtroom as a platform,'' Professor Wedgwood said. Mr. Moussaoui is representing himself at trial, and his freewheeling, conspiracy-minded litigation style has presented the court with difficult choices.

''I'm not sure we'd handle Moussaoui in the same way now,'' she said.

Mr. Padilla and Mr. Hamdi could yet be tried in federal court or before a military tribunal, though the latter would require redrafting the recently issued regulations, which exclude Americans from the tribunals' jurisdiction.

''The administration has repeatedly said it will not put American citizens before military tribunals,'' Professor Freedman said, ''and it is not going to be able to hold people locked up indefinitely with no process at all.''

Professor Wedgwood said the government's seeming preference for military detention over criminal trials in more recent cases demonstrated how its thinking was changing.

''You do have to see it as part of an evolving national time of reflection about how to handle these cases,'' she said.

Photo: In contrast to John Walker Lindh, another American, Jose Padilla, is being held by the military. (Agence France-Presse)