This Article explores the substantive and procedural aspects of the assertion that recklessness is included on the spectrum of mens rea for war crimes as a matter of customary international law. The substantive aspect of the inquiry, in Part I, engages in a critical assessment of the assertion that the jurisprudence of international criminal tribunals indicates that recklessness is sufficient to support a war crimes prosecution in general. The procedural aspect, in Part II, contests the prevailing “principal-agent” construct of describing the relationship between states and international criminal tribunals and the resulting role of tribunals in establishing customary international law. After rejecting the prevailing construct, the Article introduces the “designate and extend” model to clarify the relationship between states and international criminal tribunals. The substantive inquiry in Part I demonstrates that the jurisprudence of international criminal tribunals does indicate that recklessness is included on the mens rea spectrum for war crimes, but only in specific, limited conditions. The procedural inquiry in Part II, while applying the new designate and extend model, confirms the role of decisions by international criminal tribunals as a subsidiary — rather than primary — source of customary international law. The substantive aspect of the inquiry addresses the specific issue of the spectrum of mens rea for war crimes in order to refine the existing legal standard, while the procedural aspect adopts a broader approach to clarify the general relationship between states and international criminal tribunals. Both inquires address unsettled issues that are central to the theory and practice of public international law.