The United States has formally charged a Yemeni, described as Osama bin Laden's driver, with conspiracy to commit murder and terrorist acts as a member of al-Qaeda.
Mr Salim Ahmed Hamdan went before a panel of five US military officers for a pretrial hearing at the US naval base in Guantanamo Bay, where he has been held for more than two years. His Pentagon-appointed lawyer challenged the court's authority to try Mr Hamdan and its ability to judge impartially.
It is the first US war crimes tribunal since World War II.
The United States has charged four of the 585 al-Qaeda or Taliban suspects at Guantanamo with conspiracy to commit war crimes, a charge that carries a maximum penalty of life in prison.
The other three are also scheduled this week to go before the tribunals.
Two other defendants, Mr Ali Hamza Ahmed Sulayman al Bahlul, of Yemen, and Mr Ibrahim Ahmed Mahmoud al Qosi, of Sudan, are accused of being bin Laden bodyguards. The third defendant is Mr David Hicks of Australia, who is accused of fighting for al-Qaeda.
Human rights groups have criticised the United States for its treatment of the Guantanamo prisoners, most of whom were detained during the war in Afghanistan in late 2001 or early 2002 and have not been charged or given access to lawyers.
The United States alleges Mr Hamdan met bin Laden in Afghanistan in 1996 and became a personal driver and bodyguard for him and other high-ranking al-Qaeda members, and that he transported weapons for al-Qaeda.
Mr Hamdan's lawyer, Navy Lieutenant Commander Charles Swift, filed nearly a dozen legal challenges. He said US President George W. Bush lacked constitutional authority to order the tribunals. He said the charges should be dropped because his client had not been granted a promised hearing to determine if he had been properly classified as an enemy combatant and because he said the tribunal members had been improperly influenced by members of the Bush administration.
Mr Swift said the charge against his client did not exist as a crime under any US or international law until long after he was brought to Guantanamo.
The tribunal said it would meet again in early November to consider those issues.
The last US war crimes tribunals took place in 1942, when eight Germans were captured after sneaking into New York and Florida by submarine to blow up railroads, bridges and factories. Six were executed.