Hamdan was captured by Afghan militiamen in late November 2001 after the radical Islamic Taliban movement was driven from power in Afghanistan by U.S.-backed Afghan forces. He was subsequently turned over to U.S. authorities, who sent him to the U.S. detention facility at the Guantanamo Bay naval base in Cuba in 2002.

He acknowledged that he had worked as a bodyguard and driver for Osama bin Laden, whom he met in Afghanistan in 1996. But he denied having any role in the Sept. 11, 2001, terrorist attacks carried out by bin Laden's al-Qaeda network.

On Nov. 13, 2001 -- the day the Afghan capital, Kabul, fell to U.S.-backed forces after five years of Taliban rule -- President Bush issued Military Order No. 1 declaring that military commissions would try foreign terrorist suspects for alleged war crimes and sentence them to punishments including death. The administration argued that the commissions were authorized by laws on military justice, by a congressional resolution passed on Sept. 14, 2001, and by the powers vested in the president as commander in chief under the U.S. Constitution.

Hamdan later became one of the first 10 detainees at Guantanamo chosen to face military trials. He was charged in July 2004 with conspiracy to commit terrorism and war crimes while serving as a weapons courier and driver for bin Laden and other top al-Qaeda members. If convicted, he faced a maximum sentence of life in prison.

Military prosecutors alleged that Hamdan delivered arms, ammunition and other supplies to al-Qaeda fighters, picked up weapons at Taliban warehouses and drove or accompanied bin Laden to appearances at al-Qaeda training camps and other events. During these appearances, bin Laden would give speeches encouraging followers to carry out suicide attacks and engage in holy war against Americans, the prosecution alleged.

Specifically, prosecutors charged, Hamdan served as a driver in a convoy in which bin Laden fled potential U.S. reprisal attacks in Afghanistan at the time of the al-Qaeda bombings of two U.S. embassies in East Africa in 1998 and the attacks on the World Trade Center and Pentagon on Sept. 11, 2001. In addition, he allegedly received weapons training at al-Qaeda's Farouq training camp in southern Afghanistan on various occasions between 1996 and 2001.

In April 2004, Hamdan, represented by Georgetown University law professor Neal K. Katyal, , filed a petition for habeas corpus, challenging the legality of his detention. While the petition was pending before the U.S. District Court in Washington, the government formally filed the conspiracy charges against him and set in motion his trial before a military commission.

In August 2004, Hamdan appeared in a makeshift courtroom at Guantanamo as the U.S. military formally opened its first trial of an alleged al-Qaeda collaborator. His appearance, after nearly three years in detention, marked the first time that the United States had used military commissions to try war crimes suspects since World War II.

Hamdan's military attorney promptly attacked the military commission process, calling it unfair, and challenged the qualifications of the presiding officer and several other members.

In November 2004, the U.S. District Court granted Hamdan's habeas petition in part, ordering a halt to the military commission. The court ruled that Hamdan could not be tried by a military commission unless a competent tribunal determined that he was actually an "unlawful combatant" and not a prisoner of war under the 1949 Geneva Convention.

Hamdan maintained that instead of facing a military commission under a presidential order, he should be tried by a court martial under the U.S. Code of Military Justice in accordance with the 1949 convention. That would afford him the same rights accorded to U.S. military personnel tried by courts martial, rather than the restrictions he would encounter in a military commission. Human rights groups have charged that the commissions' rules do not meet international standards for fair trials.

The Bush administration appealed the District Court's ruling, and the Defense Department meanwhile gave Hamdan and other Guantanamo detainees hearings before a Combatant Status Review Tribunal. In Hamdan's case, the tribunal affirmed that he was an enemy combatant requiring continued detention. It said he was "either a member of or affiliated with al-Qaeda."

In July 2005, the U.S. Court of Appeals for the District of Columbia Circuit overturned the District Court's decision, breathing new life into the military commissions. The appeals court said the Geneva Convention does not apply to al-Qaeda members and that the military commissions were authorized by Congress.

The Supreme Court agreed in November last year to hear Hamdan's appeal of the ruling. Chief Justice Roberts, one of the judges who voted against Hamdan's appeal when he served on the appeals court, recused himself from the case.

Congress entered the fray in December, passing the Detainee Treatment Act, which stripped federal courts of jurisdiction over Guantanamo detainees' habeas corpus petitions that were "pending on or after" the date of the law's enactment. The act also provided an alternative military process for reviewing the enemy combatant status of detainees and designated the D.C. Circuit appeals court as the sole venue for appeals of military commission verdicts.

Arguing that the act implicitly accepts the legitimacy of the military commissions and that it disallows Hamdan's habeas petition, the administration asked the Supreme Court in January to dismiss the case. Administration lawyers said the proper time for Hamdan to file a constitutional challenge was after his trial before a military commission.