Any other findings by the court, however, could upend the commissions. Among the potential outcomes is for the court to split 4-4, because new Chief Justice John G. Roberts Jr. recused himself in the case. He was part of a three-judge appeals court panel last summer that upheld the commissions - a ruling that would stand if there were no Supreme Court majority.
Maj. Jane E. Boomer, an Air Force attorney and spokeswoman for the Office on Military Commissions, declined last week to discuss any contingency plans if the high court rules against the military commissions.
"We're cognizant of the pending decision from the Supreme Court, and we will carefully review the opinion when it is released and respond accordingly," Boomer said. "But at this time, we will not speculate on what the court is going to do."
The Bush administration would not have to automatically turn to existing courts, said Carl Tobias, a law professor at the University of Richmond.
"One solution would be for the executive to work with the Congress and fashion legislation that would clearly authorize these commissions if they are needed," Tobias said. "There are many better ways to approach it than what the administration has done by itself that would afford more due process, more openness ... and still wouldn't take every case to federal court. It's not either-or, all or nothing."
Duke's Silliman said a more likely outcome is that most of detainees will never face trial.
"I don't think they're going to be prosecuted at all," he said. "I think that probably 95 percent of the detainees at Guantanamo Bay, we have insufficient information on them to come up with a criminal charge, period. ... What we'd like to do is say, `OK, we'll send them back to France, Germany, Yemen, Saudi Arabia. You come up with some charge and keep them off the street.' And, well, they can't easily do that."
A balancing act
Past wars nearly always have included prolonged detentions of enemy fighters, with the majority released at the end of armed conflict. But the current fight against terrorism presents thornier issues that include questions about whether alleged al-Qaida fighters should be treated as prisoners of war and when to authorize the release of detainees if the terror threat is expected to stretch on without end.
Goodman of the Center for Constitutional Rights said he fears that the U.S. government will employ a system of "outsourcing illegal detention," in which detainees transferred to foreign jails could continue to be held indefinitely, without court review and at risk of torture.
Others, including Silliman, said that complicated balancing act could mean that the final chapter for Guantanamo Bay stretches well into the next administration. The military is prepared for that possibility: A new $30 million prison building at the detention center, with air conditioning, medical bays and exercise areas, is expected to be completed by August.
"Part of closing Guantanamo is to send some folks back home, like we've been doing," Bush said. "Of course, sometimes we get criticized for sending some people out of Guantanamo back to their home country because of the nature of the home countries; a little bit of a Catch-22."