ASHINGTON, Nov. 11 — In its decision to accept the Guantánamo Bay prisoners' appeals despite the Bush administration's objections, the Supreme Court brushed past the "judges keep out" fence the administration had tried to erect around its open-ended detention policy.

No matter how the court eventually rules, that action alone may well come to define a singular moment in the relationship between the White House and the Supreme Court, two inherently powerful institutions that for the last several years have been in alpha mode, each intent on exercising its power to the maximum extent possible.

Though it may not have been clear that the court was ready to join the post-Sept. 11 debate, it now appears that the administration laid down a challenge the justices were unwilling to ignore. This was a moment long in coming: the imperial presidency meets the imperial judiciary.

There were less confrontational ways for the administration to defend its view that the Guantánamo policy does not violate any constitutional or statutory rights. It could have defended the policy on its merits, taking the position that the detainees are receiving all the due process to which they are entitled under the circumstances and summoning the deference that the Supreme Court and other courts have traditionally given to executive branch claims of military necessity.

Or it could have defended the position it took successfully before the lower courts — that the federal courts have no jurisdiction over the detention of foreigners held in military custody outside the country's borders — while conceding that this jurisdictional question was itself sufficiently important to merit the Supreme Court's attention. Parties to Supreme Court cases, who have won in the lower courts, do occasionally acquiesce to the court's review while continuing to defend their victory.

Instead, the administration drew an uncompromising line at the threshold of the entire debate, insisting in the brief filed by Solicitor General Theodore B. Olson that these were not cases that the Supreme Court should even hear. The implication was that there was nothing to discuss.

Yet the question of jurisdiction — whether the courthouse doors are open to various categories of cases and claimants — goes to the heart of the Supreme Court's role, as the court's critics as well as its friends have always understood.

There have been periodic efforts in Congress to strip the federal courts of jurisdiction over questions — abortion, school busing for integration, prayer in the classroom — to which lawmakers think the courts are giving the wrong answers. Senator Wayne Allard, Republican of Colorado, recently introduced a bill to strip the lower federal courts of oversight on cases involving the Pledge of Allegiance, display of the Ten Commandments on public property and other touchy church-state questions.

Jurisdictional questions, in other words, lie not at the margins but at the core of the judicial function. The question the justices framed on Monday for their review of the Guantánamo cases — "Whether United States courts lack jurisdiction to consider challenges to the legality of the detention of foreign nationals captured abroad in connection with hostilities and incarcerated at the Guantánamo Bay Naval Base, Cuba" — may have appeared at first glance to reflect only a technical or preliminary slice of the larger debate.

But to the justices, that is the question from which all else flows. If indeed there is no jurisdiction — if a 1950 precedent, issued in a quite different but unmistakably related context, really means that the federal courts may not review the Guantánamo detentions — then it will be the Supreme Court and not the White House that says so.

The administration's argument that the Supreme Court should not even hear the cases was thus a direct challenge to the court's sense of itself, a battle joined on the court's own most sacred ground.

"I'm surprised the administration chose to defend such a hard-line position," David A. Strauss, a former assistant solicitor general who now teaches at the University of Chicago Law School, said on Tuesday. "It's almost as if they are interested in vindicating an abstract point." Mr. Strauss signed one of the "friend of the court" briefs urging the justices to accept the cases.

The administration's stance was consistent with its uncompromising position in disputes with other branches of government. It refused a Congressional request for information about the energy policy task force Vice President Dick Cheney ran early in the administration, and recently appealed to the Supreme Court to block a federal district judge's ruling that two outside groups, Judicial Watch and the Sierra Club, were entitled to some information about the task force. The administration characterizes the lower court's ruling as threatening to "violate fundamental principles of the separation of powers."

As for the Supreme Court, under Chief Justice William H. Rehnquist, it has been extremely jealous of its prerogative to "say what the law is," in the words of the chief justice's judicial hero and predecessor 12 times removed, Chief Justice John Marshall in the landmark case of Marbury v. Madison.

In a series of recent decisions, the court has made it clear that it regards constitutional interpretation as an exercise for itself alone, not to be shared with the other branches. In a recent article, two Yale law professors, Robert C. Post and Reva B. Siegel, wrote that the court's current "juricentric" view of its role departs substantially from recent tradition.

The battle over who gets the last word in this round may have little bearing on the fate of the Guantánamo detainees. Even if the court finds jurisdiction, it is highly unlikely that any federal judge would order a detainee's release over military objections. But that does not diminish the importance of what happened on Monday, when the Supreme Court could have turned away but decided, instead, to decide.