Under contemporary international law, war crimes are conceived as particularly serious violations of the laws of armed conflict. Mere participation of rank-and-file soldiers in an unjust or unlawful war is generally not considered to warrant legal punishment. This position is based on the principle of equality between belligerents. During the last 20 years, this principle has been challenged by the so-called revisionist position in just war theory, as well as by certain scholars in international law. According to them, unjust or unlawful participants in armed conflict perpetrate serious wrongs. This article argues that their conduct is not only morally wrongful, but also that it should be criminalized under certain circumstances. On the basis of empirical research on cognitive biases, and on one of the leading accounts of legitimate authority in political philosophy, it argues that participation in war warrants criminalization only when the war is knowingly or manifestly unlawful. Furthermore, it claims that this position is not only sound at the level of deep moral principles, but that in fact it provides a persuasive reinterpretation of existing international law.