- The "Detainee Treatment Act" does not divest the Supreme Court of jurisdiction to hear the case. This was a statutory-construction argument; it did not reach the constitutional question (whether the DTA, even if interpreted as the Bush Administration wishes, violates constitutional safeguards). Further, it's not a purely military matter from which the civilian courts should abstain—after all, Hamdan wasn't in the military!
- The commissions are not authorized by an act of Congress. Even under the most-expansive reading of the legislation at issue—and note that this portion of the Court's opinion is written by a dissenter in the case that posited that reading—that legislation did not give the executive branch unfettered discretion; it only acknowledged that the executive branch's existing powers under Article 21 (10 U.S.C. § 821) of the Uniform Code of Military Justice (UCMJ) might potentially apply.
- The commission procedure is inconsistent with the UCMJ. Specifically, excluding Hamdan from his own trial (among other problems) violates Article 36 (10 U.S.C. § 836), and the evidentiary regime.
- The commission procedure is inconsistent with the Geneva Conventions, for many of the same reasons… and that "war means war." This is, in essence, the most important finding concerning the procedures. Congress could act to fix the defects in items 1 through 3 if it chooses; it cannot unilaterally amend a treaty, and this treaty does not require enabling legislation.
The most interesting passage, as is often the case in these heavily policy weighted decisions by Justice Stevens, occurs very near the end.
We have assumed, as we must, that the allegations made in the Governments charge against Hamdan are true. We have assumed, moreover, the truth of the message implicit in that charge — viz., that Hamdan is a dangerous individual whose beliefs, if acted upon, would cause great harm and even death to innocent civilians, and who would act upon those beliefs if given the opportunity. It bears emphasizing that Hamdan does not challenge, and we do not today address, the Government's power to detain him for the duration of active hostilities in order to prevent such harm. But in undertaking to try Hamdan and subject him to criminal punishment, the Executive is bound to comply with the Rule of Law that prevails in this jurisdiction.
Slip op. at 72.
Justice Scalia's dissent goes only to point 1: He would have held that the DTA did abrogate the Court's jurisdiction, which should not have been exercised on prudential grounds anyway. Justice Thomas's dissent goes to the merits; he would hold that the Court's substantive opinion (points 2 through 4) "openly flouts our well-established duty to respect the Executive's judgment in matters of military operations and foreign affairs." This assertion doesn't entirely dispose of the matter, contrary to Justice Thomas's position. Does the detention of Hamdan himself fall within that purported sphere of supremacy? And, if so, does that mean that blatantly exceeding the authority granted to that sphere is unreviewable? There is an argument to be made here; Justice Thomas does not do so, or at least not clearly and satisfactorily.
Hamdan is really a matter of retail justice clashing with wholesale policy. Justice Stevens's invocation of the Rule of Law is what really separates the US from its enemies.