The detainee provisions passed as part of the National Defense Authorization Act for FY2012 (2012 NDAA; P.L. 112-81 ), affirm that the Authorization for Use of Military Force (AUMF) in response to the terrorist attacks of September 11, 2001, authorize the detention of persons captured in connection with hostilities. The act provides for the first time a statutory definition of covered persons whose detention is authorized pursuant to the AUMF. During consideration of the detention provision, much of the debate focused on the applicability of this detention authority to U.S. citizens and other persons within the United States. Congress ultimately adopted a Senate amendment to clarify that the provision is not intended to affect any existing law or authorities relating to the detention of U.S. citizens or lawful resident aliens, or any other persons captured or arrested in the United States. This report analyzes the existing law and authority to detain, as "enemy combatants," U.S. persons, which, for the purpose of this report means persons who are generally understood to be subject to U.S. territorial jurisdiction or otherwise entitled to constitutional protections; that is, American citizens, resident aliens, and other persons within the United States. In June, 2004, the Supreme Court handed down a series of opinions related to wartime detention authority. In Hamdi v. Rumsfeld , a plurality of the Court held that a U.S. citizen allegedly captured during combat in Afghanistan and incarcerated at a Navy brig in South Carolina could be held as an enemy combatant as part of the necessary force authorized by Congress after the terrorist attacks of September 11, 2001 , but that he was entitled to notice and an opportunity to be heard by a neutral decision maker regarding the government's reasons for detaining him. The government instead reached an agreement with the petitioner that allowed him to return to Saudi Arabia, where he also holds citizenship, subject to certain conditions. On the same day, the Court in Rumsfeld v. Padilla overturned a lower court's grant of habeas corpus to another U.S. citizen in military custody in South Carolina on jurisdictional grounds, sending the case to a district court in the Fourth Circuit for a new trial. The vacated decision of the U.S. Court of Appeals for the Second Circuit had held that the circumstance of a U.S. citizen arrested in the United States on suspicion of planning to carry out a terrorist attack there was fundamentally different from the case of a citizen captured on the battlefield overseas, and that the detention of such a citizen without trial was therefore precluded by the Non-Detention Act, 18 U.S.C. Section 4001(a), which provides that no U.S. citizen may be detained except pursuant to an act of Congress. A plurality of the Court found in Hamdi that the President's detention of a U.S. citizen captured on the battlefield is not foreclosed by the Non-Detention Act because an act of Congress, the AUMF, explicitly authorized such detention, but emphasized the narrow limits of the authority it was approving: The AUMF authorizes the President to use "all necessary and appropriate force" against "nations, organizations, or persons" associated with the September 11, 2001, terrorist attacks. 115 Stat. 224. There can be no doubt that individuals who fought against the United States in Afghanistan as part of the Taliban, an organization known to have supported the al Qaeda terrorist network responsible for those attacks, are individuals Congress sought to target in passing the AUMF. We conclude that detention of individuals falling into the limited category we are considering, for the duration of the particular conflict in which they were captured, is so fundamental and accepted an incident to war as to be an exercise of the "necessary and appropriate force" Congress has authorized the President to use. The plurality went on to describe the kind of detention it had in mind was the traditional practice of detaining prisoners of war under long-standing law of war principles: Further, we understand Congress' grant of authority for the use of "necessary and appropriate force" to include the authority to detain for the duration of the relevant conflict, and our understanding is based on longstanding law-of-war principles. If the practical circumstances of a given conflict are entirely unlike those of the conflicts that informed the development of the law of war, that understanding may unravel. But that is not the situation we face as of this date. Active combat operations against Taliban fighters apparently are ongoing in Afghanistan. The United States may detain, for the duration of these hostilities, individuals legitimately determined to be Taliban combatants who "engaged in an armed conflict against the United States." If the record establishes that United States troops are still involved in active combat in Afghanistan, those detentions are part of the exercise of "necessary and appropriate force," and therefore are authorized by the AUMF. Justice Souter, joined by Justice Ginsburg joined the plurality opinion to provide sufficient votes to vacate the decision below and remand the case to give Hamdi an opportunity to contest his detention. However, finding no explicit authority in the AUMF (or other statutes) to detain persons as enemy combatants, they would have determined that 18 U.S.C. Section 4001(a) precludes the detention of American citizens as enemy combatants altogether. They rejected the theory that the detention was authorized as a necessary incident to the use of military force because "the Government's stated legal position in its campaign against the Taliban ... is apparently at odds with its claim here to be acting in accordance with customary law of war and hence to be within the terms of the Force Resolution in its detention of Hamdi." In other words, the two Justices appeared to agree in principle that the AUMF could authorize the detention of prisoners of war, but took the view that the government's failure to accord the Taliban detainees rights under the Geneva Convention vitiated that authority. Justice Scalia, joined by Justice Stevens, dissented, arguing that "our constitutional tradition has been to prosecute [U.S. citizens accused of waging war against the government] in federal court for treason or some other crime" unless Congress has suspended the Writ of Habeas Corpus pursuant to the Constitution's Suspension Clause, Art. I, Section 9, cl. 2. They viewed as "unthinkable that the Executive could render otherwise criminal grounds for detention noncriminal merely by disclaiming an intent to prosecute, or by asserting that it was incapacitating dangerous offenders rather than punishing wrongdoing." Under their view, even if the AUMF did authorize detention in sufficiently clear language to overcome the prohibition in 18 U.S.C. Section 4001(a) (which, in their view, clearly it did not), Hamdi's detention would have been unconstitutional without a proper suspension of the Writ. Justice Scalia described his position as pertaining only to U.S. citizens detained within the United States (regardless of where captured), suggesting that only citizens who were concededly members of enemy forces may be detained as prisoners of war within the United States. Justice Thomas also dissented, essentially agreeing with the government's position that the detention of enemy combatants is an unreviewable aspect of the war powers constitutionally allocated to the political branches. He agreed that the AUMF provides sufficient authority to detain enemy combatants, meaning that a majority of the Court approved that position, but he would have given utmost deference to the Executive branch and accorded little in the way of due process. Finally, he questioned whether other acts of war, such as bombings and missile strikes, would also be subject to due process inquiry. Although a bare majority of the Court, led by Chief Justice Rehnquist, declined to decide in Padilla whether the detention authority approved in Hamdi would apply to a U.S. citizen arrested in the United States, four Justices who dissented on the question of jurisdiction also indicated they would have upheld the Second Circuit's grant of the petition on the merits. Apparently rejecting the Bush Administration's contention that it had the authority to detain a U.S. citizen who was alleged to be "closely associated with Al Qaeda" and to have "engaged in ... hostile and war-like acts, including ... preparation for acts of international terrorism" against the United States in order to extract intelligence and prevent him from aiding Al Qaeda, Justice Stevens, joined by Justices Souter, Ginsburg, and Breyer, wrote: Executive detention of subversive citizens, like detention of enemy soldiers to keep them off the battlefield, may sometimes be justified to prevent persons from launching or becoming missiles of destruction. It may not, however, be justified by the naked interest in using unlawful procedures to extract information. Incommunicado detention for months on end is such a procedure. Whether the information so procured is more or less reliable than that acquired by more extreme forms of torture is of no consequence. For if this Nation is to remain true to the ideals symbolized by its flag, it must not wield the tools of tyrants even to resist an assault by the forces of tyranny. Given Justice Scalia's dissent in Hamdi , it appeared in 2004 that a majority of the Supreme Court as it was then constituted would have determined that the Non-Detention Act precludes the detention of a U.S. citizen without trial based on an alleged association with Al Qaeda and participation in a terrorist plot far from any conventional battlefield, at least within the United States. A separate majority of the same Court took the view that the Non-Detention Act does not preclude the detention of a U.S. citizen picked up on the battlefield in Afghanistan, albeit apparently for different reasons. There also appears to have been a majority on the Court who believed that indefinite detention solely for the purpose of interrogation would be impermissible even where they agreed the law of war supports detention.