More than two years ago, Supreme Court Justice Sandra Day O’Connor cautioned that “a state of war is not a blank check for the President.” The United States Congress, apparently, never got the memo. Last week, lawmakers passed a bill that hands the President sweeping detention and interrogation powers while eliminating habeas corpus review for any noncitizen he labels an “enemy combatant.”

The Supreme Court’s June decision in Hamdan v. Rumsfeld provided the impetus for the new law, known as the Military Commissions Act of 2006 (MCA). Hamdan challenged the President’s makeshift system of military commissions for trying suspected terrorists of war crimes. The Court said that the commissions failed to satisfy basic requirements of US military law and international law because, among other things, they denied defendants the right to be present at trial and to see the evidence against them.

Even more important, the Hamdan ruling rejected a central premise of the Administration’s “war on terror” by finding that members of Al Qaeda are protected, at a minimum, by Common Article 3 of the Geneva Conventions. In other words, even suspected terrorists are not beyond the law. Besides prohibiting trials by kangaroo courts, Common Article 3 outlaws torture, cruel treatment and “outrages upon personal dignity.” Hamdan‘s ruling on Common Article 3 has already had significant ramifications: It not only halted the President’s military commissions but also prompted the Administration to shut down its secret CIA detention centers and transfer fourteen high-level Al Qaeda suspects to Guantánamo.

Former Solicitor General Walter Dellinger described Hamdan as the Supreme Court’s most important separation of powers decision ever. Perhaps, but therein lies the nub. By telling the President that he had exceeded the limits imposed by Congress, the Court left open the possibility that Congress would remove those limits. A group of Republican senators led by John Warner, John McCain and Lindsey Graham may have succeeded in preventing the President from openly violating the Geneva Conventions, but they ultimately agreed to a “compromise” that guts existing enforcement mechanisms by, for example, eliminating judicial review over violations of the Geneva Conventions.

The fight now shifts from Capital Hill to the courts. First and foremost, judges must address whether Congress has revoked habeas corpus jurisdiction.

Here, the MCA will run up against the Constitution itself. The Framers intended the Constitution’s guarantee of habeas corpus, known as the Suspension Clause, to serve as a check on the Executive and on Congress. They sharply restricted the circumstances under which lawmakers can deny prisoners meaningful access to the courts by suspending the writ. Habeas corpus may be suspended, the Constitution states, only in cases of “rebellion or invasion,” when it is required to preserve public safety.

No one who drafted the Constitution raised the slightest objection to the Suspension Clause. America, after all, had not revolted from English tyranny only to create a new regime where individuals could be imprisoned by executive decree. The only debate among the Framers of the Constitution was whether Congress could ever suspend the writ, a question resolved by limiting suspensions to true emergencies.

Since the nation’s founding in 1789, the writ has been suspended on only four occasions, most memorably by President Lincoln during the Civil War. (It was also suspended amid active rebellion in the Reconstruction South and the Philippines in the early 1900s, and in Hawaii during World War II.) Each suspension was carefully limited in duration to the necessity that prompted it. Never before has Congress eliminated habeas corpus for a vast class of individuals (any noncitizen) in amorphous conflict (the global “war on terror”) that could last generations.

It is always difficult to predict how the Supreme Court will rule, but the signs are encouraging. In 2004 the Court decided in Rasul v. Bush that Guantánamo detainees can challenge their detention by habeas corpus. While Justice O’Connor has retired, the other five Justices who voted for habeas corpus review remain on the Court, including Justice Kennedy, who separately underscored his concerns about creating legal black holes. Moreover, the same five Justices who ruled for Hamdan will likely not be pleased at the latest assault on what Chief Justice John Marshall once described as the Court’s unique function: to say what the law is.

The MCA will also be tested in the next frontier of litigation over US detainee policy: the growth of other offshore prisons, such as Bagram Air Base in Afghanistan. After the Supreme Court decided Rasul, the government stopped bringing prisoners to Guantánamo because of its concern that judges would take a hard look at the facts, including who was imprisoned there and how they were treated. Whether the Supreme Court will decide that the Constitution guarantees review over detentions at places like Bagram is uncertain. Otherwise, the Court will do precisely what Justice O’Connor cautioned against: create a perverse incentive for the Administration to imprison individuals without judicial review.

For five years, the President has strived to avoid all restrictions by creating lawless enclaves like Guantánamo. Torture is condemned publicly but practiced privately under the name of “enhanced” interrogation techniques. Individuals are imprisoned as “enemy combatants” even though they are neither enemies nor combatants. Thus far, the Supreme Court has repeatedly rebuked the Executive and cabined presidential power. With any hope, the Court will continue to do so, resisting the naked power grab that the Administration calls the “war on terror.”