The ruling, a rebuke to the administration and its aggressive anti-terror policies, was written by Justice John Paul Stevens, who said the proposed trials were illegal under U.S. law and international Geneva conventions.
The case focused on Salim Ahmed Hamdan, a Yemeni who worked as a bodyguard and driver for Osama bin Laden. Hamdan, 36, has spent four years in the U.S. prison in Cuba. He faces a single count of conspiring against U.S. citizens from 1996 to November 2001.
Two years ago, the court rejected Mr. 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.
That decision rejects the Bush administration's claims that al Qaeda prisoners aren't protected under the Geneva accords, reports CBS News correspondent Wyatt Andrews. But the ruling gives Mr. Bush a way out, suggesting he use the court martial system or go back to Congress.
Thursday's ruling overturned that decision.
"Any way you slice it, this is a defeat for the Bush Administration, which wanted the go-ahead from the Court to begin prosecuting these detainees under military procedures that government lawyers had come up with," CBS News legal analyst Andrew Cohen says. "Now the White House has to go to Plan B, but I'm not sure the folks there know what that even looks like."
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CBS News White House correspondent Peter Maer reports the White House was stunned by the ruling. Speaking after a meeting with Japan's visiting prime minister Thursday, President Bush said he will work with Congress to get approval to try terrorism suspects before military tribunals.
Today's ruling "won't cause killers to be put out on the street," Mr. Bush said, and that he "will not jeopardize the safety of the American people."
The president also noted that since the ruling came in during that meeting, he hadn't had a chance to fully review it.