Background Briefing

Definition of Unlawful Enemy Combatant

7.  What is the definition of “combatant” under international law?

International humanitarian law (the laws of war) draws a distinction between “combatants,” defined as members of armed forces, and civilians. Civilians who are actively participating in hostilities can be treated as combatants. Deeming someone to be a combatant has important consequences: under international humanitarian law, combatants may be lawfully attacked and killed, and are subject to capture.

8.  What is the definition of “unlawful enemy combatant” under the MCA and how does it comport with international law?

The MCA expands the definition of “combatant” to include those who have “purposefully and materially” supported hostilities against the United States, even if they have not taken part in the hostilities themselves, and even if they are arrested far from the battlefield. This turns ordinary civilians – such as a mother giving food to her combatant son, an individual who sends money to a banned group, or a U.S. resident who commits a criminal act unrelated to armed conflict – into “combatants” who can be placed in military custody and hauled before a military commission.

An additional – and circular – provision specifies that anyone who has been determined to be an “unlawful enemy combatant” by a Combatant Status Review Tribunal (the military boards convened to allow detainees at Guantanamo Bay to contest their status as combatants, called CSRTs) or “another competent tribunal” established by the president or the defense secretary is presumed to be an enemy combatant for the purposes of military commissions. This provision does not include any substantive criteria to guide the deliberations of such tribunals. And, notably, the definition of enemy combatant that has been used by the CSRTs at Guantanamo is even broader than the definition contained in the legislation, encompassing even the unknowing financier of a charitable arm of a terrorist organization. In at least one known case, a CSRT labeled a detainee an enemy combatant for precisely that reason.1

These definitions have essentially been invented by the administration and Congress. They have no basis in international law and undermine one of the most fundamental pillars of the Geneva Conventions – the distinction between combatants, who engage in hostilities and are subject to attack, and non-combatants. 

9.  Does the legislation authorize the indefinite detention of anyone who falls within its definition of “unlawful enemy combatant”?

The MCA does not explicitly address the question of detention. Yet in detaining persons as enemy combatants, the administration may point to the definition of unlawful enemy combatant under the MCA to try to justify who may be so detained.

Senators John McCain, John Warner, and Lindsay Graham – three of the primary authors of this legislation – have argued that this definition “simply establishes the jurisdiction of military commissions” and does not, in any way, authorize the arrest and indefinite detention of those who fall within this broad category.2 The legislation itself specifies that the definition of “unlawful enemy combatant” applies to the section of the U.S. code establishing military commissions.

1 Combatant Status Review Tribunal Transcript of Detainee ISN #229, U.S. Department of Defense, Set 4, Page 390, Released March 3, 2006,

2 John Warner, John McCain, Lindsay Graham, “Looking Past the Tortured Distortions”, Wall Street Journal¸ October 2, 2006.