While Congress is expected to adopt a new Authorization for Use of Military Force (AUMF) against the Islamic State of Iraq and the Levant (ISIL), debate continues on the scope of a new AUMF, including how to describe the entities against which force would be authorized.

A number of AUMF proposals would authorize force against both ISIL and “associated” or “affiliated” forces. President Obama’s recently proposed draft AUMF authorizes the use of force against ISIL or “associated persons or forces,” which it defines as “individuals and organizations fighting for, on behalf of, or alongside ISIL or any closely-related successor entity in hostilities against the United States or its coalition partners.”

The term “associated forces” is not new. In a 2012 speech on the 2001 AUMF, then-Department of Defense General Counsel Jeh Johnson explained that the notion of an ‘associated force’ is based on the “concept of co-belligerency in the law of war.” He stated that an ‘associated force’ has two characteristics: it is (1) an organized, armed group that has entered the fight alongside al Qaeda, and (2) a co-belligerent with al Qaeda in hostilities against the United States or its coalition partners. As Jack Goldsmith, Ryan Goodman, and Steve Vladeck recently noted, the element of “‘co-belligerency’ is a concept that has been put forward by the administration in litigation and accepted by federal courts.” This blog post briefly explores the concepts of “belligerency” and “co-belligerency” under the international law of neutrality, which traditionally applies to States.

The international law of neutrality

The international law of neutrality regulates relations between States that are participating in an international armed conflict (belligerent States) and those that are not (neutral States). A neutral State must defend its rights (for instance by preventing belligerents from committing violations of its territorial integrity), remain impartial towards belligerents, and refrain from participating in the conflict.

A neutral State violates neutrality by breaching its obligation to remain impartial and to not participate in the conflict. For instance, a State would violate neutrality by supplying warships, arms, ammunition, military provisions or other war materials, either directly or indirectly, to a belligerent, by engaging its own military forces, or by supplying military advisors to a party to the armed conflict. Allowing belligerent use of neutral territory as a military base, the storage of war material or passage of belligerent troops or munitions in neutral territory, furnishing troops to a belligerent, or providing or transmitting military intelligence on behalf of a belligerent are also examples of violations of neutrality.

A State also violates its neutrality if it establishes on its territory communication channels for a party to the conflict or places telecommunication installations (such as a military communications system) at the disposal of a belligerent when these installations would not be available to them in normal conditions. Massive financial support for a party to the conflict, through gifts or loans, also constitutes a violation of neutrality.

Nevertheless, violating neutrality does not necessarily bring an end to neutrality, and must not be confused with the beginning of belligerent status. While a neutral State’s violation of the law of neutrality may elicit countermeasures, it does not suffice to render it a party to an armed conflict.

The beginning of belligerency and co-belligerency under the international law of neutrality

A State’s neutrality will end when the armed conflict ends or when the State becomes a party to an armed conflict, or, in other words, a belligerent. A State becomes a belligerent under the law of neutrality by (1) declaring war; (2) participating in hostilities to a significant extent; or (3) systematic or substantial violations of its duties of impartiality and non-participation.

For instance, a 2004 Memo of the U.S. Office of Legal Counsel written by then-Assistant Attorney General Jack Goldsmith explored the type of conflict participation required for a State to qualify as a belligerent or co-belligerent:

mere participation in any aspect of the occupation itself will not always suffice to constitute co-belligerency, especially when a State’s specific contribution has no direct nexus with belligerent or hostile activities. … But a State that sends military forces to assist in rounding up Baathist remnants and imposing general security in Iraq, and especially one that participates in hostile activities in Iraq, will engage in conduct properly characterized as belligerent. In sum, the determination whether a State is a “co-belligerent” by virtue of its participation in the occupation of Iraq turns on whether the participation is closely related to ‘hostilities.’ (emphasis added)

If significantly participating in hostilities or systematically or substantially violating duties of impartiality and non-participation will transform a neutral State into a belligerent, then what type of relationship will render the State a co-belligerent of a party to the armed conflict?

Author Morris Greenspan has proposed that a co-belligerent is a “fully fledged belligerent fighting in association with one or more belligerent powers.” According to the UN Special Rapporteur on extrajudicial, summary or arbitrary executions, “Co-belligerency is a concept that applies to international armed conflicts and entails a sovereign State becoming a party to a conflict, either through formal or informal processes. A treaty of alliance may be concluded as a formal process, while an informal process could involve providing assistance to or establishing a common cause with belligerent forces.”

In sum, the systematic or substantial supply of war materials, military troops, or financial support in association, cooperation, assistance or common cause with another belligerent would make it a co-belligerent.

Applying the co-belligerency concept as a guide to determine which nonstate armed groups fall within the AUMF, Curtis Bradley and Jack Goldsmith have proposed that “Terrorist organizations that act as agents of al Qaeda, participate with al Qaeda in acts of war against the United States, systematically provide military resources to al Qaeda, or serve as fundamental communication links in the war against the United States, and perhaps those that systematically permit their buildings and safehouses to be used by al Qaeda in the war against the United States, are analogous to co-belligerents in a traditional war.” These examples appear to mirror the type of behavior that would amount to a State’s co-belligerent status.

The co-belligerency concept has been widely criticized as too broad in the application of the 2001 AUMF, as it potentially sweeps in too many formal and informal groups operating in various countries. Yet President Obama’s proposed AUMF definition of “associated forces” is even broader and omits even the bare limits that the notion of co-belligerency provides.

Nevertheless, if, for domestic purposes of applying an AUMF, the meaning of “associated forces” is still to be informed by, or assumed to be consistent with, the international law concept of co-belligerency traditionally applicable to States, then, for the analogy to fully apply, an associated force would have to participate in hostilities to a significant extent or otherwise participate in the armed conflict in a systematic or substantial manner, and would need to do so with a sufficient nexus to ISIL.

Of course, an AUMF definition should have no effect on the U.S.’s continuing obligation to comply with international law in the use of force. In a non-international armed conflict, it can be lawful to attack members of an organized armed group of a party to the conflict, but debate persists as to whether one may target only those individuals who have a “continuous combat function” or rather any member of the group. Persons who do not fall within this category are civilians and are entitled to protection against direct attack unless and for such time as they directly participate in hostilities.


* This blog post is based on a longer paper prepared as background for a recent roundtable for government and independent experts co-hosted by the International Committee of the Red Cross and Columbia Law School’s Human Rights Institute. Principal sources on the law of neutrality include: M. Bothe, The Law of Neutrality, in Handbook of International Humanitarian Law 571 (D. Fleck ed., 2d ed., 2008); E. Castrén, The Present Law of War and Neutrality (1954); L. F. L. Oppenheim, 2 International Law: A Treatise (H. Lauterpacht, ed., 1940); and M. Greenspan, The Modern Law of Land Warfare (1959).