Constitutionally, America's founding fathers entrusted the president with the primary responsibility, and therefore the power, to ensure the security of the US in situations of "grave and unforeseen emergencies". It goes on: "Numerous presidents have ordered the capture, detention, and questioning of enemy combatants during virtually every major conflict in the nation's history, including recent conflicts in Korea, Vietnam and the Persian Gulf". And it continues: "Congress can no more interfere with the president's conduct of the interrogation of enemy combatants than it can dictate strategy or tactical decisions on the battlefield."
http://www.guardian.co.uk/usa/story/0,12271,1237239,00.html

The document states that US interrogators can use harsh measures as long as they were not "specifically intended" to inflict "severe mental pain or suffering". In another passage, it says that even if an interrogator "knows that severe pain will result from his actions, if causing harm is not his objective, he lacks the requisite specific intent."