President Barack Obama's support for preventively detaining terrorism suspects undoubtedly surprised some of his longtime backers. Holding prisoners at Guantanamo, without the certainty of trial or release, was a defining feature of the previous administration's counterterrorism policy – and some of its fiercest critics expected Obama to change the policies.

But the possibility had been percolating for months. With his pledge in January to close the Guantanamo prison within a year, Obama set off a fierce, mostly under-the-radar debate among legal experts about whether it will be possible to meet the goal he announced yesterday: to build "a legitimate legal framework" for imprisoning terrorism suspects indefinitely.

The question affects more than Guantanamo. The fates of 169 detainees there remain undecided, according to Obama's numbers yesterday, and administration officials have suggested that they will be unable to prosecute as many as 100. But the legal status of thousands more held by the United States in Afghanistan and elsewhere overseas also hangs in limbo, and any detention policy will have ongoing effects as the fight against al-Qaida continues.

Here are some of the key issues facing the architects of a new preventive detention system, or, as it's sometimes called, a "national security court":

President Obama said yesterday that some suspected terrorists "cannot be prosecuted." How could that be – haven't there been plenty of previous cases involving terrorism?

Yes, there have been. A significant number of people have been convicted of terrorism-related offenses in federal trials, including several accused of acting on behalf of al-Qaida. From September 2001 to May of last year, the government won 145 convictions against terrorism suspects, according to an analysis by former federal prosecutors for the progressive legal nonprofit, Human Rights First.

That's not to say it's easy. The criminal justice system was built to safeguard the rights of defendants. Prosecutors can't win a case without enough admissible evidence. Sometimes, as in this week's arrest of four suspects in New York, investigators have tape-recordings of the alleged illegal activity. But more often than not, they depend on witnesses. They can't use testimony obtained through abusive interrogations. And intelligence agencies are typically loath to collaborate with a public prosecution that puts their sources on a witness stand.

Recent court filings by the Obama administration in cases challenging the legality of Guantanamo detentions offer a glimpse of possible hurdles to prosecuting an accused terrorist.

Criminal defendants have the right to see information in government files that could help show their innocence, so prosecutors have a duty to search for all plausibly relevant documents to turn over. Officials said in the filings that evidence about the Guantanamo detainees tops 1.8 million pages, total. All of those would need to be searched for exculpatory information. A test query for several detainees yielded between several hundreds and tens of thousands of "hits" each.

Isn't detention without trial illegal on its face?

Not necessarily. The traditional laws of war permit preventive detention of both enemy soldiers and hostile civilians until the end of the fight. Standards like the Geneva Conventions require humane treatment of these detainees.

Holding aggressors without any intention of trying them is a time-honored right of fighting nations. Why? In wartime, combatants are supposed to fight – so, fighting itself is not a crime. Fighting dirty – for instance, purposefully killing innocent civilians – is prosecutable as a war crime. But even then, there's no right to a speedy trial, and the captor nation can take its time deciding when or even whether to press charges. It has long been accepted that a nation at war has the right to protect itself by keeping enemies from returning to the battlefield, without having to invest resources or risk public release of military secrets in full-blown trials.

Every trial risks the possibility that a defendant could be acquitted or receive a moderate sentence. Under the laws of war, governments don't need to take that risk.

The possibility that convicted terrorists could win relatively quick release isn't just theoretical. Of the three military commission convictions so far at Guantanamo, two resulted in sentences of, essentially, time served. One of those convicted was Salim Hamdan, Osama bin Laden's driver, for providing material support to al-Qaida. He was sentenced last year to five-and-a-half years in prison – six months more than time served – and now lives free in Yemen, in a case where the government had sought life imprisonment.

A detention system premised on the laws of war would permit Obama to keep his promise of yesterday: "We are not going to release anyone if it would endanger our national security."

So is the United States at war with terrorists?

A trickier question than it might seem. Terrorists don't wear uniforms or rush to a battlefield. The front, many argue, could be anywhere – a hotel room in Albania or an alley in Manila.

For its part, al-Qaida declared war on the United States in 1998, shortly before its operatives blew up American embassies in Kenya and Tanzania. Obama has expressed no skepticism on this point, saying in his speech: "We are indeed at war with al-Qaida and its affiliates." He was able to say so because Congress, which has the constitutional power to declare war, issued the 2001 Authorization for Use of Military Force after the 9/11 attacks.

That means some terrorists can be held indefinitely as prisoners of war, according to David Cole, a Georgetown University law professor long associated with civil liberties causes. Cole recently stunned the progressive legal community by supporting preventive detention for some detainees in a Boston Reviewessay.

Cole explained in an interview, "You might not have evidence that would satisfy the criminal-conviction standard of proof beyond a reasonable doubt, but perhaps you have very good evidence that a person was a fighter for the Taliban. Should we just release him to go back to the caves and start shooting at U.S. soldiers, just because we don't have sufficient proof to convict him of a crime?"

Retired U.S. Army Major General John Altenburg, who until resigning in November 2006 had the task of deciding which Guantanamo detainees would be slated for military commission trials, said the Bush administration's "arrogance and naiveté" about public perception had tarnished the otherwise valid notion of detaining terrorism suspects under a wartime rationale. He said in an interview, "What the previous administration did was allow critics to define the terms of the debate to be the terms of domestic criminal law. So the public is reacting with, ‘What about their lawyer? What about their right to a speedy trial?'"

In fact, Altenburg said, the al-Qaida detainees are not entitled to a speedy trial any more than German prisoners of war in World War II were.

If wartime detention is OK, and the U.S. is at war with terrorists, then why does the nation need a new detention law?

This appears to be as much a question of political support as one of legal reasoning.

Obama said yesterday that he'd seek a law spelling out procedures for preventative detention for reasons of political legitimacy. He said he wanted to avoid his predecessor's "ad hoc legal approach for fighting terrorism."

Earlier this week, top Obama aides invited the most ardent opponents of preventive detention, including the head of the American Civil Liberties Union, to a two-and-a-half-hour meeting. Although administration officials have not publicly discussed that session, some guests were startled by the argument that the president already has sufficient authority to preventively detain terrorism suspects. Kenneth Roth, the executive director of Human Rights Watch, said afterward on a conference call with reporters that there was a "surprising misapprehension about what the laws of war permit."

The legal authority that courts have recognized for the current military detentions of "enemy combatants" is the 2001 Authorization for Use of Military Force, which doesn't even mention detentions. If captives are moved to U.S. soil, they'll likely be able to invoke greater legal protections than they've got now, according to a January analysis by Congressional Research Service lawyers. Possibly, some will even be able to seek political asylum under immigration laws. Long-term preventive detention would therefore require a new law and possibly amendments to others.

Part of the legal puzzle has to do with trying to apply traditional laws of war to the "novel" type of conflict that is terrorism, says Harvey Rishikof, professor of law and national security studies at the National War College. It's just harder to tell who's a combatant – and therefore detainable as a POW – and who's a criminal suspect due for trial, because terrorists are "stateless actors" eschewing uniforms and avoiding battlefields.

Attorney General Eric Holder hinted at the complexity of the "battlefield" question as it applies to terrorist combatants at his confirmation hearing. "There are physical battlefields, certainly, in Afghanistan," he said. "But there are battlefields, potentially, you know, in our nation. There are cyber battlefields." He went on, "There's a battlefield, if you want to call it that, with regard to the hearts and minds of the people in the Islamic world."

Major General Altenburg said, "I personally think the battlefield has to be beyond the ground of an Afghanistan, because al-Qaida is everywhere. Now, there's no [court] holding anywhere that says that is the law of war, because again, this is unprecedented."

How would preventive detention of terrorism suspects work?

The closest the public has gotten to a legislative blueprint for preventive detention of terrorism suspects appeared in a recent Wall Street Journalop-ed by Republican Sens. Lindsey Graham and John McCain. They called for a "uniform set of standards and procedures administered by a civilian judge," who would decide the challenges to the legality of detention that the Supreme Court has said are a detainee's right, and "an annual interagency review" to determine whether a detainee continues to threaten national security and should be held. The senators are expected to be influential voices as any new policy develops.

But before looking at the procedures, policymakers will have to decide who will face detention. The Bush administration initially claimed that it could indefinitely detain anyone the executive branch deemed an "enemy combatant." The courts trimmed back that sweeping view, saying that the authority was shared with Congress and subject to judicial review.

The question is especially acute for terrorism detention, says Harvard law professor and former Bush official Jack Goldsmith, who with Neal Katyal – then a Georgetown law professor, now Obama's principal deputy solicitor general – was one of the earliest proponents of a new legal regime for terrorism suspects. Because this enemy doesn't wear a uniform and, to the contrary, takes pains to blend with civilians, identifying candidates for military detention is harder. But since the end of this conflict may similarly be hard to know, there's a risk that wrong decisions could harm innocent people for a very long time, Goldsmith warns.

One way to get lawmakers to seek utmost accuracy in any detention system, said Goldsmith at a recent Brookings Institution forum, would be to apply it to U.S. citizens as well. "The threat of terrorism can come as easily from a U.S. citizen," he said. He noted, though, that the idea "is controversial and probably a nonstarter." It could also be struck down by the Supreme Court, where there are some strong views that citizenship comes with special constitutional protections.

Cole, the Georgetown law professor, stressed that only detainees fitting a classic war-captive profile – members of an organization against whom Congress has authorized the use of military force, who deliberately act or plan harm in order to advance the military goals of the enemy – should be considered for preventive detention. Other terrorists, he said, "should be dealt with through the criminal law." Broadening the field, he said, would be "a first step on a slippery slope of a broader use of preventive detention for other crimes."

Some opponents of preventive detention say it's Orwellian that such a system would imprison a person based on future dangerousness. But, says Cole, even if "we can't predict the future," it is possible to measure whether there is "a substantial risk that someone will engage in future dangerous conduct. Waiting for a wrong is not adequate." Such judgments are made all the time, he said, in civil commitment proceedings, bail hearings and immigration decisions. He said that the key was to focus the inquiry narrowly, not on suspects' character or beliefs, but on "whether they pose a risk of returning to the battle."

Retired U.S. Navy Lieutenant Commander Charles Swift, who as a military lawyer represented Hamdan in his commission trial, worries that a preventive detention option will allow inaccurate judgments of dangerousness. He said that, after hearing all the evidence, "military [jurors] didn't view Mr. Hamdan as a substantial war criminal." But someone like Hamdan would be a prime candidate for indefinite detention over prosecution, he said: "The indicia of his criminality were extremely low, but his proximity to bin Laden was extremely high. The adversary system helped show how Hamdan was not dangerous – the question is, whether a national security court would allow that."

Crucial questions will have to be answered about what burden of proof the government would have to meet to put someone in preventive detention. In civil trials, the prevailing party has to win by a preponderance of the evidence – meaning it's more likely than not that the party is right. The criminal standard of proof beyond a reasonable doubt is much higher. Cole advocates that preventive detention be permitted if the government shows "clear and convincing evidence" that the detainee fits certain dangerousness criteria – the current standard for deciding whether an ordinary criminal suspect can be released on bail.

Proponents agree the government should have to periodically renew its case for detaining a person. While wartime detention permits imprisonment until the end of hostilities, no one assumes that it will be clear when hostilities with al-Qaida and its affiliates have ended. Holder said at his confirmation hearing that he could see such reviews happening annually.

Any new system will also have to build in rights for people facing detention. Should they have lawyers of their own choosing? Can hearsay evidence – the testimony of people who don't have to show up in court and answer for themselves – be accepted against them, as it is under certain exceptions in ordinary court proceedings?

Should detention hearings be open or secret? Federal courts are permitted to seal documents or close sessions in cases involving classified or other sensitive information. The presumption, however, is that the courts are open. Goldsmith said preventive detention proceedings should also be presumed public, calling it "essential" to establishing legitimacy here and abroad.

Finally, there is the question of who will decide. The Supreme Court has said that a neutral decision-maker is required to ensure due process for detainees. One of many criticisms of Guantanamo's Combatant Status Review Tribunals – instituted by the Bush administration and still in effect today – is that the decision-makers are subordinates of the very military commanders who claim the detainees should be held as enemy combatants. Most current proposals for a new detention system say independent federal judges should make the final call.