The only surviving World War II veteran on the Supreme Court, Justice John Paul Stevens, appointed three decades ago by a President as Republican as W., delivered the plain and airtight message: President Bush violated every standard of the military code, the US Constitution and international law with his order for military tribunals at Guantánamo. In its implications if not always its direct findings, Hamdan v. Rumsfeld is to Bush what the Pentagon Papers case was to Richard Nixon: a devastating rebuke to a President who thought he had a blank check; a clear reaffirmation of the rule of law even–or especially–in times of national crisis.

The Court’s Hamdan ruling emphatically does not shut Guantánamo down. Indeed, the Court majority took pains to assert that the attacks of September 11 ignited the President’s war powers and they do not challenge “the Government’s power to detain [Salim Ahmed Hamdan] for the duration of active hostilities.”

The ruling unambiguously declares that the President may not simply invent trials that conform to no known standard of law, which are not necessitated by urgent battlefield conditions, and deny defense lawyers access to evidence. It also dismantles every element of the Administration’s case, from the conspiracy-to-commit-war crimes charges against the Yemeni national who was Osama bin Laden’s driver in Afganistan to the necessity of an improvised process governed by no act of Congress. “Any urgent need…is utterly belied by the record,” Justice Stevens writes. “Hamdan was arrested in November 2001 and he was not charged until mid-2004. These simply are not the circumstances in which, by any stretch of the historical evidence or this Court’s precedents” justify a drumhead military commission.

In particular, Justice Stevens’ majority ruling deals a devasating blow to tribunal rules which violate the Uniform Code of Military Justice and the Geneva Convenions. Indeed, the most signficiant news in Justice Steven’s Hamdan majority ruling is fierce insistence on the power of international law, and in particular the Geneva Conventions, which the Administration has long dismissed as irrelevant to non-state actors like Al Qaeda volunteers. Such dismissals are nonsense, according to Justice Stevens’ ruling: the Geneva Conventions’ Common Article 3 clearly prohibits “the passing of sentences…without previous judgment…by a regularly constituted court affording all the judicial guarantees … recognized as indispensable by civilized peoples.” For three years, Administration lawyers have argued that the Geneva Conventions don’t apply to its “war on terror”. That argument is finished.

The Administration was probably prepared to lose the Hamdan tribunals. It is not clear, however, that the White House is ready for the sweeping implications of the Supreme Court’s firm invocation of internationally recognized human rights standards. Black sites, secret interrogations, torture, the whole panoply of lawless methods embraced by Bush and Rumsfeld now stand exposed.

At bottom, the Hamdan ruling is what legal scholar Jack M. Balkin calls “democracy-forcing”: It restores checks and balances, strips the President of illegally-seized powers, and requires the President to go back to Congress for an open debate on any new tribunals he would like to establish, as well as any revision to the nation’s adherence to international law. What Justice Stevens and the five-vote Court majority have done is raise the floor on human rights–not just for Bin Laden’s driver and not just at Guantánamo, but in Washington itself.