Hamdan v. Rumsfeld
Hamdan v. Rumsfeld is a 2006 decision of the Supreme Court of the United States. It denied the government position that an alien captured, during combat outside the United States and turned over to U.S. forces, could be tried by military commission. The Court denied the U.S. government interpretation of the Detainee Treatment Act, "DTA §1005(e)(1) provides that “no court … shall have jurisdiction to hear or consider … an application for … habeas corpus filed by … an alien detained … at Guantanamo Bay”", on the grounds that the law did not apply ex post facto to pending cases. Further, they rejected the argument that this was appropriate under Schlesinger v. Councilman, which allowed military adjudication for the purpose of maintaining good military order and discipline, citing Ex parte Quirin as more applicable because it dealt with agents of a hostile power. "Absent a more specific congressional authorization, this Court’s task is, as it was in Quirin, to decide whether Hamdan’s military commission is so justified." The Court found there was no legislative authorization or precedent for such justification..
During the active combat phase of the Afghanistan War (2001-), Afghan militia captured Hamdan, a Yemeni national, and turned him over to the U. S. military. They, in 2002, imprisoned him in the Guantanamo detention camp. President George W. Bush, over a year later, determined Hamdan was an enemy combatant to be tried by military commission, for "then-unspecified crimes. After another year, he was charged with conspiracy 'to commit … offenses triable by military commission.'" 
Further, the court held that there was no legislative authority, under either the Authorization for the Use of Military Force or the Detainee Treatment Act, for this specific military commission. They interpreted Quirin as asserting Article of War 15, which authorized military commissions for war crimes. UCMJ Article 21 is essentially widentical to Article of War 15, and states, “The jurisdiction [of] courts-martial shall not be construed as depriving military commissions … of concurrent jurisdiction in respect of offenders or offenses that by statute or by the law of war may be tried by such … commissions.”
Quirin, in the opinion of the Court, was not "a sweeping mandate for the President to invoke military commissions whenever he deems them necessary. Rather, Quirin recognized that Congress had simply preserved what power, under the Constitution and the common law of war, the President already had to convene military commissions—with the express condition that he and those under his command comply with the law of war.... Neither the AUMF nor the DTA can be read to provide specific, overriding authorization for the commission convened to try Hamdan."
Several other laws and cases bear on the complex situation of a conflict involving the allegiance of the defendant, citizenship, and the place where captured. The Posse Comitatus Act forbids the use of the Army for civilian law enforcement, but the argument here was that the capture was purely a part of militry operations outside the United States, where civilian law does not apply. Ex parte Quirin established military jurisdiction over individuals captured on the soil of the United States, but who were operating for Nazi Germany during a declared war, even if some were U.S. citizens.
Johnson v. Eisentrager was argued by the government, but it had significant differences, among them being that the petitioners were citizens of a nation that had been in a state of declared war with the United States.
The term enemy combatant is a variant on the criteria for being eligible for prisoner of war (POW) status under the Third Geneva Convention. The more common language is "unlawful combatant", a lawful combatant meeting the criteria, adjudicated by a "competent tribunal" if necessary, for POW eligibility.
Another argument for Presidential authority for military commissions is an interpretation of the Authorization for the Use of Military Force, as interpreted in Hamdi v. Rumsfeld. According to the Court, however, nothing in the AUMF’s text or legislative history suggested Congress intended to expand or alter the authorization set forth in UCMJ Art. 21. No such authority can be inferred from the Detainee Treatment Act, which has no language authorizing that tribunal or any other at Guantanamo Bay. "Together, the UCMJ, the AUMF, and the DTA at most acknowledge a general Presidential authority to convene military commissions in circumstances where justified under the Constitution and laws, including the law of war."
To some extent, the Military Commissions Act of 2006 showed Congressional objection to this decision, although there were other issues both of Congressional authority and simply trying to clarify. 
- Hamdan et al. v. Rumsfeld, Secretary of Defense, et al.', 415 F. 3d. John Paul Stevens, 33 (Supreme Court of the United States June 29, 2006)
- The Articles of War were the predecessor to the Uniform Code of Military Justice (UCMJ)
- Jennifer Elsea (October 29, 2001), Trying Terrorists as War Criminals, Congressional Research Service, CRS Order Code RS21056
- "The Need to Roll Back Presidential Power Grabs", New York Review of Books 56 (8), May 14, 2009