WASHINGTON - The Supreme Court declared Thursday that President Bush overstepped his authority in the war against terrorism, ruling that he does not have the power to set up special military trials at Guantanamo Bay without the approval of Congress.

In a 5-3 decision, the high court said the planned military tribunals lack the basic standards of fairness required by the nation's Uniform Code of Military Justice and by the Geneva Conventions.

The ruling is the most sweeping legal defeat for the administration in the five-year-old war on terrorism, and it rejects the president's broad claim that the commander in chief can make the rules during an unconventional war.

Since 1929, the Geneva Conventions have set rules for the conduct of wars and the treatment of prisoners, but Bush and his top advisers had maintained that it did not apply to terrorists.

Still, the practical impact of Thursday's decision may be limited. The court said al-Qaeda suspects could be tried under the rules for courts-martial used by the American military or under new rules passed by Congress.

The ruling does not free any terrorism suspects, nor does it change the status of the several hundred detainees at the detention camp at Guantanamo Bay, Cuba.

The opinion in Hamdan v. Rumsfeld was delivered by 86-year-old Justice John Paul Stevens, the court's last veteran of World War II, who set forth a view of the Constitution in wartime that stood in sharp contrast to the one put forth by the president and his lawyers. Justices Stephen H. Breyer, Anthony M. Kennedy, David H. Souter and Ruth Bader Ginsburg agreed.

Justices Antonin Scalia, Clarence Thomas and Samuel A. Alito Jr. dissented. Scalia and Thomas took the rare step of reading their dissents in the courtroom. Chief Justice John G. Roberts Jr. took no part in the decision because he was on the U.S. Court of Appeals last year when the issue was considered and voted then to uphold the president's special military trials.

In the decision, Stevens said the Constitution gives Congress the power to make the laws and set the rules for handling wartime captives. It says Congress shall "make rules concerning captures on Land and Water," and also says Congress shall define the "offenses against the law of nations."

Despite those words, Bush contended that as commander in chief of the armed forces, he had the power on his own to decide how terrorism suspects would be held, how they were to be treated, how they would be tried and what offenses amounted to war crimes.

In November 2001, the White House issued an executive order announcing the Pentagon would set up special military commissions to try al-Qaeda suspects. The president said that he did not need the approval of Congress and that the federal courts had no jurisdiction over the cases.

Moreover, the White House said the Geneva Conventions did not apply to terrorists because this was not a conflict between nations and their armies.

No one has been tried and convicted under the new rules, but they were challenged as unfair by Salim Ahmed Hamdan, a one-time driver for Osama bin Laden who was charged with conspiring with al-Qaeda to kill Americans. Hamdan, who was captured in November 2001 and has been held at Guantanamo since June 2002, has admitted he was bin Laden's driver but said he was a $200-a-month hired hand, not a terrorist.

In the sweeping decision, the justices rejected all key assertions made by the president and struck down the military commissions set up by the Pentagon. The court also cast doubt on whether the general charge of conspiracy that Hamdan faces is a war crime.

Civil libertarians hailed the ruling as a repudiation of the president's willingness to set aside established U.S. and international laws in the war against terrorism. "The Supreme Court's decision reaffirms the importance of one of this country's founding principles: Trials conducted in the name of the United States must be full, fair and according to law," said Deborah Pearlstein, a lawyer for Human Rights First.

As Stevens noted, the Bush administration's rules would have allowed the use of evidence obtained through coercion, and they could have resulted in a defendant and his lawyer being excluded from his trial.

Bush said he plans to work with lawmakers. "To the extent that there is latitude to work with the Congress to determine whether or not the military tribunals will be an avenue in which to give people their day in court, we will do so," he said.

Two key Senate Republicans, Lindsey Graham of South Carolina and Jon Kyl of Arizona, said they were disappointed with the decision. "However, we believe the problems cited by the court can and should be fixed," they said in a joint statement. "We intend to pursue legislation ... granting the executive branch the authority to ensure that terrorists can be tried by competent military commissions."

One part of the court's decision suggests the Geneva Conventions protect captured terrorists, even though they are not regular soldiers and not entitled to be treated as prisoners of war. Stevens described Article 3 of the Geneva Conventions as applying to persons who are caught up in military conflicts, even if they are not soldiers. If captured and charged with crimes, these persons must be tried "by a regularly constituted court affording all the judicial guarantees which are recognized as indispensable by civilized peoples," Article 3 says.

The Geneva Conventions did not define a "regularly constituted court," but the five justices agreed such a tribunal must meet "the standards of our military justice system."

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