WASHINGTON, June 28 — Declaring that "a state of war is not a blank check for the president," the Supreme Court ruled on Monday that those deemed enemy combatants by the Bush administration, both in the United States and at Guantánamo Bay, Cuba, must be given the ability to challenge their detention before a judge or other "neutral decision-maker."

Although divided in its rationale, the court was decisive in rejecting the administration's core legal argument that the executive branch has the last word in imposing open-ended detention on citizens and noncitizens alike. The justices' language was occasionally passionate, reflecting their awareness of the historic nature of this confrontation between executive and judicial authority.

Eight justices, all but Justice Clarence Thomas, said the two-year-long detention of an American citizen, Yaser Esam Hamdi, had either been invalid from the beginning or had become so, for constitutional or statutory reasons. The controlling opinion, by Justice Sandra Day O'Connor, said that Mr. Hamdi's detention was permissible if designation as an enemy combatant proved to be correct, but that his inability so far to appear before a judge, challenge the government's evidence, and tell his side of the story had deprived him of his constitutional right to due process.