Geof Stone on Guantanamo
Nations at war have always had the legal authority to detain captured enemy soldiers ("prisoners of war") to prevent them from returning to the battlefield. Similarly, the U.S. has the legal authority to detain captured "enemy combatants" in the War on Terror in order to ensure the safety of the nation.
Central to the legality (and morality) of this authority, though, is the determination that the person detained is, in fact, an enemy soldier or combatant. In conventional warfare, this is usually easy, because soldiers wear uniforms. In the War on Terror, however, enemy combatants do not wear uniforms. This is a problem, because it requires us to determine in some fair and reasonable manner whether particular individuals are in fact affiliated with the enemy. It would be unjust and counter-productive for us to detain people who are not actually a threat to us but were innocently swept up in the inevitable chaos of war.
In part for this reason, the Supreme Court has held that individuals detained at Guantanamo have the right to habeas corpus -- that is, they have the right to ask a federal court to determine whether they are being lawfully held. Put simply, the military cannot legally detain an individual at Guantanamo unless it can show by a "preponderance of the evidence" that he is in fact an enemy combatant.