A brief filed by the plaintiff in the recently-dismissed Al-Aulaqi v. Obama demonstrates the importance of the concept of "direct participation in hostilities" (DPH) in international humanitarian law (IHL), especially since September 11, 2001. The nature and scope of direct participation has been at the forefront of debates about the legitimacy of U.S. actions - such as targeted killings in its ongoing armed conflict with al-Qaeda, the Taliban, and associated forces - in the IHL community.

In the absence of state response, the International Committee for the Red Cross's (ICRC) 2009 study Interpretive Guidance on the Notion of Direct Participation in Hostilities Under International Humanitarian Law (the Interpretive Guidance) is becoming the authoritative guidance on defining and interpreting DPH for the international community - despite extensive criticism from leading commentators. Given that state practice and opinio juris, rather than ICRC publications, should be the primary sources informing the development of customary international law (CIL), the status of this document is unfortunate. Until a prominent war-fighting state such as the United States directly participates in this important discussion by publishing an official response, however, the Interpretive Guidance will increasingly be viewed as binding custom.

This Essay will begin with an introduction to the concept of DPH in international humanitarian law, followed by a summary of the Interpretive Guidance and the criticisms leveled against it. It will then argue that the United States must officially respond to the Interpretive Guidance due to the ICRC's unique role in shaping customary international law; the important nexus between direct participation in hostilities and ongoing U.S. military operations; and the need for legal legitimacy in conducting those operations. Having demonstrated that the United States must respond, this Essay will make the case that the upcoming U.S. Department of Defense (DOD) Law of War Manual is the best vehicle for doing so.