Seven years ago, the Pentagon began imprisoning men it described as "very hard cases," "the worst of the worst" among terrorists in American custody, at the U.S. Naval Base in Guantanamo Bay, Cuba. Since then it has released more than 500 of them. "What's left," Vice President Dick Cheney declared last week, "is the hard core." That was right before the Pentagon released half a dozen more.

Unless the Bush administration recklessly loosed hundreds of hardened terrorists on the world, the president's men evidently were mistaken when they said every detainee belonged in that category. That pattern of error reinforces the argument against allowing the executive branch to wield the kind of unchallengeable authority it asserted at Guantanamo.

As President Obama proceeds with his plan to close the prison, he should recognize that Guantanamo is not so much a place as a state of mind. It's an attitude that says: We know who the bad guys are, and we're not about to let anyone endanger national security by second-guessing us.

The Bush administration manifestly did not know who the bad guys were. Its methods for identifying "unlawful enemy combatants," defined as anyone, anywhere who belonged to or supported the Taliban or Al Qaeda, were sloppy and haphazard.

More than 90 percent of the 779 men held at Guantanamo were captured not by Americans but by Afghan militiamen, Pakistani forces or other parties of dubious reliability, often in anticipation of bounties the United States had promised. Many detainees were either minor hangers-on or entirely innocent, held based on the uncorroborated word of self-interested captors or of prisoners eager to please interrogators who used "enhanced" techniques to extract accusations.

The Pentagon acknowledges that 17 Chinese Muslims it has held since 2002 were incorrectly identified as unlawful enemy combatants but says it cannot send them back to China because they might be persecuted there. At the same time, it has appealed a federal judge's order to release them in the United States.

Haji Bismullah, one of the men freed over the weekend, fought the Taliban and later served as a regional transportation official in Afghanistan's pro-American government. After members of a rival clan who coveted his position accused him of terrorist connections, he was held at Guantanamo for nearly six years before a military panel, belatedly paying attention to the witnesses who vouched for him, decided he "should no longer be deemed an enemy combatant."

Since the Supreme Court ruled last June that Guantanamo detainees may pursue habeas corpus petitions in federal court, the government has lost 23 of 26 cases. The most recent one involved Mohammed el Gharani, a Chadian who was detained by Pakistani forces at a Karachi mosque in 2001, when he was 14.

Last week, U.S. District Judge Richard Leon ordered Gharani's release, finding that the case against him was based almost entirely on the unconfirmed, inconsistent accounts of two prisoners whose reliability the government itself had questioned. Among other things, Gharani was accused of belonging to a London-based Al Qaeda cell in 1998, when he was 11 and living in Saudi Arabia.

In November, Leon ruled that the government did not have enough evidence to detain five Algerians who were arrested in Bosnia in 2001 and accused of (SET ITAL) intending (END ITAL) to fight with the Taliban. He found that the charge was based "exclusively on the information contained in a classified document from an unnamed source."

Leon, a Bush appointee who ruled in 2005 that Guantanamo detainees could not pursue habeas corpus claims, probably was inclined to side with the government. Furthermore, under the standard he applied, the Bush administration only had to show by "a preponderance of the evidence" (a likelihood of more than 50 percent) that the prisoners' detention was appropriate.

These cases therefore speak volumes about the fallibility of the executive branch and the need for independent review of its detention decisions. I hope our new president is listening.