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The Supreme Court delivered a blow to the Bush administration's anti-terror policies Thursday when it ruled that the president was out of line when he ordered military war-crimes trials for some Guantanamo Bay detainees.

Justice John Paul Stevens wrote the opinion, which said the proposed trials were illegal for 10 foreign terror suspects under U.S. law and Geneva conventions without congressional approval.

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A huge question in this case, Hamdan v. Rumsfeld, et al, was whether the Geneva Conventions applied to prisoners held at Guantanamo Bay. The Bush administration argued that these detainees were not prisoners of war and therefore, not eligible for treatment under the Geneva agreement.

Justice Clarence Thomas wrote a strongly worded dissent, saying the court's decision would "sorely hamper the president's ability to confront and defeat a new and deadly enemy."

The court's willingness, Thomas said, "to second-guess the determination of the political branches that these conspirators must be brought to justice is both unprecedented and dangerous."

The case focused on Salim Ahmed Hamdan, a Yemeni who worked as a bodyguard and driver for Usama bin Laden. Hamdan, 36, has spent four years in the U.S. prison at Guantanamo. He faces a single count of conspiring against U.S. citizens from 1996 to November 2001 and was defended by a government-appointed military lawyer with an assist from a private law firm.

Click here to read the Supreme Court's ruling (pdf).

Click here to read Hamdan's case against the government (pdf).

Click here to read the government's case against Hamdan (pdf).

The administration said foreign terror suspects don't have the right to come into U.S. courts and demand all of the rights afforded to U.S. citizens under the legal system here but that they would be given some rights under rules for the tribunals. The justices said conspiracy was not an appropriate charge under the so-called "laws of war," under which the administration said it could set up the tribunals.

Bush: 'I Want to Find a Way Forward'

While acknowledging that he has not yet had time to fully review the decision, Bush told reporters Thursday that he will take the court's decision "very seriously" and will work with Congress to determine whether passing legislation setting up military tribunals is a valid option.

"The American people need to know this ruling, as I understand it, won't cause killers to be put out on the street," Bush said during a joint press conference with Japanese Prime Minister Junichiro Koizumi "I was told this was not going to be the case … one thing I'm not going to do is jeopardize the safety of the American people."

Bush continued: "I want to find a way forward. I have told the people I would like there to be a way to return people from Guantanamo to their home countries but some of the people need to be tried in our courts."

Breyer said the ruling applies only to Hamdan and the possibility of his appearing before a military commission for trial, and 'nothing' in it prevents the president from approaching Congress to seek the authority he thinks is necessary to set up such courts.

Senior administration officials repeatedly said Thursday that Breyer's words were an "invitation" to the administration to approach Congress in search of revised legislative language that would make commissions at Guantanamo legally acceptable, and they pointed out that all of the constititional issues raised by Hamdan and his lawyers were roundly rejected by the court.

"The court recognized that military commisions would be appropriate as long as procedures would be consistent with [Thursday's] decision," one official said. But "we are not headed in any particular direction so far."

The officials stressed that the decision was not specific and does not affect the cases of any other detainee, the future of the Guantanamo detention facility, or the president's ability to hold people like Hamdan in a time of war.

"The opinion did not indicate a constiutional impediment to the establishment of commissions," one said.

Two years ago, the court rejected Bush's claim to have the authority to seize and detain terrorism suspects and indefinitely deny them access to courts or lawyers. In this follow-up case, the justices focused solely on the issue of trials for some of the men.

The vote was split 5-3, with moderate Justice Anthony Kennedy joining Justices Ruth Bader Ginsburg, Stephen Breyer, David Souter and Stevens in the majority. Justices Samuel Alito, Clarence Thomas and Antonin Scalia voted in the minority.

Chief Justice John Roberts, named to lead the court last September by Bush, was sidelined in the case because as an appeals court judge he had previously backed the government over Hamdan.

Thursday's ruling overturned that decision.

In his opinion, Breyer said, "Congress has not issued the executive a 'blank check."'

"Indeed, Congress has denied the president the legislative authority to create military commissions of the kind at issue here. Nothing prevents the president from returning to Congress to seek the authority he believes necessary," Breyer wrote.

But Scalia wrote in his dissent thatthe Detainee Treatment Act of 2005 is specific in its language barring any domestic court from hearing Guantanamo Bay cases.

'These Are Dangerous People'

Senior administration officials say they have been "intensely gaming out" what each type of ruling might mean for the War on Terror, for Guantanamo Bay, and for trials, over the past few days. As the White House continues to review the ruling, one senior official said this in no way means the facility is "closing tomorrow." But as Bush has said before, there is a push to get other countries to take back detainees and to try others.

"This ruling now sets a number of things into action," one official said.

Pentagon spokesman Bryan Whitman would not comment on the substance of the decision, but he did say Guantanamo Bay serves as an important detention and intelligence gathering facility. About 450 detainees are still being held at the facility, which was opened in Cuba in January 2002.

"These are dangerous people," Whitman said about those being held there. "Many have vowed to go back to the battlefield if released."

The administration had hinted in recent weeks that it was prepared for the court to set back its plans for trying Guantanamo detainees.

The president also has told reporters, "I'd like to close Guantanamo." But he added, "I also recognize that we're holding some people that are darn dangerous."

The court's ruling says nothing about whether the prison should be shut down, dealing only with plans to put detainees on trial.

"Trial by military commission raises separation-of-powers concerns of the highest order," Kennedy wrote in his opinion.

Ret. Army Maj. Gen. Robert Scales, who just returned from Guantanamo Bay, said every government branch needs to be on the same page as to how to deal with terror suspects in the United States.

"The American people and the Supreme Court and the rest of people in the enlightened world ... have to decide for themselves, are we in a state of war or are we not in a state of war?" Scales said. "The enemy is using our confusion about the conditions in the world today to their advantage and ultimately, we're going to end up with innocent dead in Europe, the United States and elsewhere in the world."

Joe Reeder, a former Army undersecretary during President Clinton's term in the White House, said there is definitely a need for separate legal proceedings for terror suspects not considered formal prisoners of war. He said the problem posed by Thursday's ruling is "easily solved" if Congress will "fix this" by passing legislation authorizing the tribunals.

"This is a really difficult situation because the fact of the matter is, historically, all civilized countries have recognized the need for the specialized tribunals," Reeder told FOX News. "Evidence gathered on the battlefield is gathered by soldiers, not trained detectives and policeman … this is soldiers grabbing stuff and throwing it in a Humvee."

The prison at Guantanamo Bay, erected in the months after the Sept. 11, 2001, terror attacks on the United States, has been a flash point for international criticism. Hundreds of people suspected of ties to Al Qaeda and the Taliban — including some teenagers — have been there since 2002.

Three detainees committed suicide there this month, using sheets and clothing to hang themselves. The deaths brought new scrutiny and criticism of the prison, along with fresh calls for its closing.

FOX News' Bret Baier, Megyn Kendall, Ian McCaleb, Liza Porteus, Nick Simeone and The Associated Press contributed to this report.