One of the most important questions in the current Guantanamo detainee litigation is whether the United States may prosecute individuals in military commissions for offenses that are not recognized as war crimes under international law. The United States maintains that such offenses-particularly, material support for terrorism and conspiracy-are violations of the`US common law of war' , a form of customary domestic liability in armed conflict distinct from international law. This paper offers a critique of the US theory from the perspective of international criminal law practice and theory. In particular, it explains how the US position unduly expands the conception of war crimes liability, thereby distorting the meaning of a war crime and undermining the value of prosecuting conduct on that basis.