It reversed the dismissal by a lower court of a habeas corpus petition brought on behalf of Yaser Esam Hamdi, a U.S. citizen who was being detained indefinitely as an illegal enemy combatant after being captured in Afghanistan in 2001.

[6] The Bush administration claimed that because Hamdi was caught in arms against the U.S., he could be properly detained as an enemy combatant,[7] without any oversight of presidential decision making, and without access to an attorney or the court system.

The administration argued that this power was constitutional and necessary to effectively fight the War on Terror, declared by the Congress of the United States in the Authorization for Use of Military Force passed after the September 11 terrorist attacks.

Judge Doumar found the government's evidence supporting Hamdi's detention "woefully inadequate," and based predominantly on hearsay and bare assertions.

Because it was "undisputed that Hamdi was captured in a zone of active combat in a foreign theater of conflict," the Fourth Circuit said that it was not proper for any court to hear a challenge of his status.

Hamdi was represented before the Court by the late Federal Public Defender Frank W. Dunham, Jr., and the Government's side was argued by the Principal Deputy Solicitor General, Paul Clement.

O'Connor suggested the Department of Defense create fact-finding tribunals similar to the AR 190-8 to determine whether a detainee merited continued detention as an enemy combatant.

Justice O'Connor also limited the reach of the Court's conclusion regarding the executive authority to detain enemy combatants: For purposes of this case, the enemy combatant that [the government] is seeking to detain is an individual who, it alleges, was part of or supporting forces hostile to the United States or coalition partners in Afghanistan and who engaged in an armed conflict against the United States there.

Justice O'Connor wrote: [W]e necessarily reject the Government's assertion that separation of powers principles mandate a heavily circumscribed role for the courts in such circumstances.

Absent suspension of the writ by Congress, a citizen detained as an enemy combatant is entitled to this process.Justice David Souter, joined by Justice Ruth Bader Ginsburg, concurred with the plurality's judgment that due process protections must be available for Hamdi to challenge his status and detention, providing a majority for that part of the ruling.

[T]he practical effect of this provision was that imprisonment without indictment or trial for felony or high treason under §7 would not exceed approximately three to six months.Justice Clarence Thomas was the only justice who sided entirely with the executive branch and the Fourth Circuit's ruling, based on his view of the security interests at stake and the President's broad war-making powers.

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.

... A citizen, no less than an alien, can be "part of or supporting forces hostile to the United States or coalition partners" and "engaged in an armed conflict against the United States," ...; such a citizen, if released, would pose the same threat of returning to the front during the ongoing conflict [as an alien].... Because detention to prevent a combatant's return to the battlefield is a fundamental incident of waging war, in permitting the use of "necessary and appropriate force", Congress has clearly and unmistakably authorized detention in the narrow circumstances considered here.

With regard to the second purpose, the plurality held "Necessary and appropriate force" amounted to authorization to detain "for the duration of the relevant conflict," in order to prevent enemy combatants from rejoining the fight.

[8] Of the four justices outside the plurality, Justices Ginsburg and Souter limited their opinions to their position that Section 4001(a) of Title 18 of the United States Code (the Non-Detention Act; enacted to prevent the sort of detention that occurred when the United States placed Japanese-American citizens in concentration camps during World War II), prevented the detention of U.S. citizens.