This was the Hamdi v. Rumsfeld decision. In this case, an American citizen was being detained in Afghanistan, then held in Guantanamo Bay detention center as an "enemy combatant." Once it was discovered he was a US citizen, a suit was filed to have his detention declared unconstitutional on the grounds that a speedy trial was not being granted. The Supreme Court found, in an 8-1 decision that, even though Congress has the ability to authorize a detention, the 5th Amendment requires that a citizen be allowed to contest the detention.
