In this case, the Supreme Court ruled that the Bush Administration had violated the law when it set up the system of military tribunals that was being used to try detainees at the Guantanamo Bay prison in Cuba.
The Court ended up holding, by a 5-3 vote, that the system was illegal. It justified this in two ways. First, it said that the military tribunals violated US military law (the Uniform Code of Military Justice). Second, it said that the system violated the Geneva Conventions, to which the United States is a signatory. For these reasons, the tribunals were not legal.