Excerpt from the Opinion by Justice Stevens:
Petitioner Salim Ahmed Hamdan, a Yemeni national, is in custody at an American prison in Guantanamo Bay, Cuba. In November 2001, during hostilities between the United States and the Taliban (which then governed Afghanistan), Hamdan was captured by militia forces and turned over to the U.S. military. In June 2002, he was transported to Guantanamo Bay. Over a year later, the President deemed him eligible for trial by military commission for then-unspecified crimes. After another year had passed, Hamdan was charged with one count of con-spiracy "to commit . . . offenses triable by military com-mission." App. to Pet. for Cert. 65a.Read the full text of the Opinion [PDF] with concurrences by Justices Breyer and Kennedy and dissents by Justices Scalia, Thomas and Alito.
Hamdan filed petitions for writs of habeas corpus and mandamus to challenge the Executive Branch's intended means of prosecuting this charge. He concedes that a court-martial constituted in accordance with the Uniform Code of Military Justice (UCMJ), 10 U. S. C. Â§801 et seq.(2000 ed. and Supp. III), would have authority to try him. His objection is that the military commission the President has convened lacks such authority, for two principal reasons: First, neither congressional Act nor the common law of war supports trial by this commission for the crime of conspiracy â an offense that, Hamdan says, is not a violation of the law of war. Second, Hamdan contends, the procedures that the President has adopted to try him violate the most basic tenets of military and international law, including the principle that a defendant must bepermitted to see and hear the evidence against him.
The District Court granted Hamdan's request for a writ of habeas corpus. 344 F. Supp. 2d 152 (DC 2004). The Court of Appeals for the District of Columbia Circuit reversed. 415 F. 3d 33 (2005). Recognizing, as we did over a half-century ago, that trial by military commission is an extraordinary measure raising important questions about the balance of powers in our constitutional structure, Ex parte Quirin, 317 U. S. 1, 19 (1942), we granted certiorari. 546 U. S. ___ (2005).
For the reasons that follow, we conclude that the military commission convened to try Hamdan lacks power to proceed because its structure and procedures violate both the UCMJ and the Geneva Conventions. Four of us also conclude, see Part V, infra, that the offense with which Hamdan has been charged is not an "offens[e] that by . . . the law of war may be tried by military commissions." 10 U. S. C. Â§821.