"The War Crimes Act of 1996, as amended, makes it a criminal offense to commit certain violations of the law of war when such offenses are committed by or against U.S. nationals or Armed Service members. Among other things, the act prohibits certain violations of Common Article 3 of the 1949 Geneva Conventions, which sets out minimum standards for the treatment of detainees in armed conflicts 'not of an international character [...]. In the 2006 case of Hamdan v. Rumsfeld, the Supreme Court rejected the Bush Administration's long-standing position that Common Article 3 was inapplicable to the present armed conflict with Al Qaeda. As a result, questions have arisen regarding the scope of the War Crimes Act as it relates to violations of Common Article 3 and the possibility that U.S. military and intelligence personnel may be prosecuted for the pre-Hamdan treatment of Al Qaeda detainees. As amended by the Military Commissions Act of 2006 (MCA, P.L. 109-366), the War Crimes Act now criminalizes only specified Common Article 3 violations labeled as 'grave breaches.' Previously, any violation of Common Article 3 constituted a criminal offense. [...]. These statutory protections, along with a number of other available defenses, appear to make it unlikely that U.S. personnel could be convicted under the War Crimes Act for any authorized conduct which was undertaken with the reasonable (though mistaken) belief that such conduct was legal. In the 110th Congress, legislative proposals were introduced to modify the scope of the War Crimes Act, and it is possible that new legislative proposals will be introduced in the 111th Congress."
CRS Report for Congress, RL33662