NEW YORK — The “war on terror” has forced democracies to grapple with the extent to which they can afford to protect the civil rights and liberties of both their citizens and foreigners. The debate has been most intense in the United States, where the refrain that the U.S. Constitution is not a “suicide pact” and that national security can justify extraordinary measures is heard repeatedly.

Some measures — unauthorized searches of bank records and wiretapping of telephone calls — compromise the liberty of all. Others — most notoriously, the confinement of roughly 450 alleged Muslim fighters at Guantanamo Bay — impinge on people thought to be the enemy.

Amid rising allegations of abuse, President George W. Bush’s government realized some time ago that it could not maintain its Guantanamo detention camp forever. Yet it did not want to repeat the experience of the Zacharias Moussaoui trial, in which, after many propagandistic appeals from the dock, the alleged 20th 9/11 hijacker was finally convicted and sentenced to life imprisonment. So the Bush administration proposed a middle way: a military commission under military judges that would recognize fewer rights for the accused and bar appeals to civilian courts.

In its recent decision in Hamdan v. Rumsfeld, though, the U.S. Supreme Court said “no”: Bush’s exercise of executive power went too far. The decision will have a lasting impact on America’s constitutional structure.

Salim Ahmad Hamdan served as Osama bin Laden’s personal driver. Other than drive his boss around and attend meetings, he did nothing more to promote the terrorist attacks of September 2001. Yet his driving and his knowledge of al-Qaida’s purposes struck the military as sufficient to charge him with entering a conspiracy to kill civilians and engage in terrorists acts.

When his trial began, the prosecution tried to exclude Hamdan from the room during the testimony of a witness against him. The government wanted to protect its sources. But this unusual procedure, permitted in the military commissions, runs afoul of accepted canons of federal, constitutional and international law. The defense immediately sought an injunction in federal court to prevent the trial from continuing.

The federal court ruled that under the Geneva Conventions, Hamdan, as a prisoner of war, was entitled to a court hearing that followed the same procedures as one judging soldiers of the detaining power — that is, an American court martial. The Supreme Court, however, sidestepped the question of whether Hamdan was a prisoner of war, arguing instead that the conflict was governed by common Article III in all the Geneva Conventions, which regulates armed conflict “not of an international character.”

In such cases, there is a duty to guarantee that all sentences are passed “by a regularly constituted court affording all the judicial guarantees which are recognized as indispensable by civilized peoples.” Bush’s military commissions were found to fall below this standard.

While the discussion of the Geneva Conventions and procedural rights is important, the more profound portion of Justice John Paul Stevens’ majority opinion addresses the boundaries of the law of war and, specifically, whether the conspiracy charge against Hamdan constitutes a violation of this body of customary international law.

Everyone agrees that, under U.S. law, the jurisdiction of military tribunals is limited to violations of the laws of war, so if conspiracy cannot be charged, Hamdan and others like him cannot be tried before any type of military court. To resolve the question of whether conspiracy constitutes a crime under the laws of war, Justice Stevens followed the arguments made in the brief that I submitted on behalf of Experts on Conspiracy and the Law of War.

The law of war, the court reasoned, must reflect the consensus of all legal systems. But conspiracy, as a stand-alone offense, is particular to Anglo-American common law. This argument has been well known since the Nuremberg trials, which rejected charges of conspiracy to commit war crimes and crimes against humanity. Justice Stevens cited Telford Taylor, the chief prosecutor in Nuremberg, who made the same point about the oddity of conspiracy charges in international tribunals.

Moreover, arguments about international law in the postwar period have quietly dropped all references to conspiracy charges. This admittedly has not been an explicit process, but the pattern is undeniable. As Justice Stevens wrote, “The crime of ‘conspiracy’ . . . does not appear in either the Geneva Conventions or the Hague Conventions — the major treaties on the law of war.” He could have added the Rome Statute and the U.N. resolutions establishing the ad hoc tribunals for the former Yugoslavia and for Rwanda.

The international community’s rejection of conspiracy charges is correct, I believe, because international criminal proceedings are oriented to post hoc justice — to judging those who are alleged to have already brought about massacres and other major crimes that concern the international community.

Conspiracy, by contrast, is suited to a legal system that emphasizes early police intervention, before criminal plans are carried out. Early intervention is possible in a domestic legal system, but there is no international police force that could fulfill a similar function.

The Bush administration is now left in an embarrassing position. It does not want to prosecute the Guantanamo detainees in federal courts, but it is unlikely that it can prosecute them under the law of war as interpreted in the Hamdan decision.

Although Hamdan recognizes the legitimacy of confining enemy combatants until the end of hostilities, doing so is untenable. The international community will not accept this form of massive preventive detention, with continuing allegations of abuse and torture. Sooner or later, Bush will have to decide between prosecutions in federal court or closing the camp.

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