Under the Military Commissions Act recently approved by Congress, there are two ways the government can imprison a suspected terrorist for the rest of his life. It can try him before a military commission, a process that includes many of the safeguards offered by civilian courts and courts-martial. Or it can skip the trial and keep him locked up anyway.

Given the latter option, the government is apt to try only its strongest cases, involving big bad guys such as Khalid Shaikh Mohammed. The result is that the best treatment will be reserved for the "worst of the worst," while the rest of those detained as "unlawful enemy combatants" -- the innocent as well as the guilty -- will be left to languish in obscurity.

The new trial procedures, though problematic in some ways, are a clear improvement over the Bush administration's original rules, which the Supreme Court overturned because they were not authorized by Congress. But these protections are available only to the detainees the government decides to try.

The rest, including the vast majority of current detainees, can be imprisoned indefinitely based on the findings of Combatant Status Review Tribunals, which perfunctorily confirm the government's detention decisions.

The Military Commissions Act does not seem to require even this pro forma review. It says an "unlawful enemy combatant" is a person who is not part of a country's uniformed armed forces but "who has engaged in hostilities or has purposefully and materially supported hostilities against the United States." It does not say who makes that determination or what evidence, if any, is required.

Alternatively, an unlawful enemy combatant is anyone so labeled by a Combatant Status Review Tribunal, which can apply its own definition. The act apparently would permit a tribunal to decide a detainee must be an unlawful enemy combatant because he's named Muhammad or because he has a beard.

The law bars detainees who are not U.S. citizens from challenging their detention in federal court, so they have no legal recourse outside the executive branch. The government can arrest "aliens," including legal visitors and residents, and hold them indefinitely, based on nothing more than the president's unilateral determination that they qualify as unlawful enemy combatants.

To recognize the danger of giving the executive branch this kind of unreviewable power, you need look no further than the men sent to Guantanamo Bay because they were falsely identified as Al Qaeda or Taliban hangers-on by Afghan warlords hungry for bounty money. Or Maher Arar, the Canadian our government mistook for an Al Qaeda member based on bad intelligence and shipped off to Syria, where he was tortured and imprisoned.

Given the elastic definition of "unlawful enemy combatant," such mistakes could morph into detention of anyone seen, rightly or wrongly, as impeding the war on terror. Whatever your assessment of George W. Bush's character, do you trust future presidents to exercise this open-ended power conscientiously and unerringly?

The Supreme Court has ruled that U.S. citizens held as unlawful enemy combatants have a constitutional right to contest their status before a "neutral decision maker." The Bush administration says a Combatant Status Review Tribunal qualifies, but so far it has been unwilling to test that argument in court. Instead it let one American detainee go and transferred the other to civilian custody for trial.

The Due Process Clause, which applies to "persons" under our government's control, does not mention nationality. When the Supreme Court ruled that non-citizens at Guantanamo have a statutory right to challenge their detention in federal court, it left open the question of whether this is also a constitutional right and, if so, whether Congress can selectively suspend it.

Since Congress has chosen to confirm the president's power grab instead of checking it, the Supreme Court will decide whether this is a country where people can be snatched off the street as enemies of the state and never be heard from again.