In a fiery hearing yesterday, one judge said that if President George W. Bush had the power to hold the trials, there was nothing to stop him from going to a US town and arresting any foreigner and denying him a trial in a US civil court.
Justice Stephen Breyer said: "If the President can do this, well, then he can set up commissions to go to Toledo, and in Toledo pick up an alien and not have any trial at all except before that special commission."
Osama bin Laden's former driver, Salim Ahmed Hamdan, who is imprisoned in Guantanamo Bay with Hicks and has been charged with conspiracy, is appealing against the legality of the military commissions, arguing they breach the Geneva Convention.
If his appeal is upheld, it would mean none of the 490 suspected terrorists held at Guantanamo Bay could be tried before military commissions.
Their cases would likely be sent to civilian courts.
Yesterday, government lawyers argued that Mr Bush had the power to convene military commissions, which have been held in the past at times of war as war crimes tribunals.
But Justice Breyer questioned the argument by Solicitor-General Paul Clement.
"You want to try a war crime. You want to say this is a war crimes tribunal," Justice Breyer said.
"One, this is not a war -- at least not an ordinary war.
"Two, it's not a war crime, because that doesn't fall under international law.
"And three, it's not a war crime tribunal or commission, because (there is) no emergency."
US government lawyers also argued that the recently passed Detainee Treatment Act, which severely restricts detainees' access to US courts, also prevented Hamdan from mounting a legal challenge to the commissions in the Supreme Court.
That argument was contested by several Supreme Court judges.
The Supreme Court is expected to make its ruling in June or July.