Editor's note: This piece is the second installment in a mutli-blog series building on the Fifth Annual Transatlantic Workshop on International Law and Armed Conflict, as explained in detail here.

The shape-shifting nature of today’s organized armed groups and their opportunistic associations with other armed groups means that a state fighting one non-state armed group (NSAG) sometimes finds itself confronting other NSAGs as well. How should we determine when two or more NSAGs are interacting in a way that unites them in a single non-international armed conflict (NIAC) against a state?

The more extensive the interactions we require among NSAGs before concluding that two NSAGs have “merged” their fight, the less easily a state will be able to treat its struggles with those groups as a single NIAC. Relatedly, the higher the law sets the bar for merging the activities of two NSAGs, the more the state fighting the NSAGs will have to assess the violence emanating from each group separately in considering whether each cluster of violence is sufficiently sustained on its own to meet the Tadic “sustained hostilities” threshold necessary to construe a situation as an armed conflict.

The Relationship Between Non-State Armed Groups

Assume State A finds itself in a NIAC with a NSAG – call it “Group X.” What happens if and when another NSAG – call it “Group Y” – begins to provide certain assistance to Group X? At what point does Group Y become part of the State A/Group X NIAC, and thus become subject to military force by State A? This question has arisen in a variety of scenarios, including in the interactions between core al Qaeda and al Qaeda in the Arabian Peninsula and between al Qaeda and al Shabaab.

Black-letter international law does not answer the question, and states and scholars have taken different approaches. What follows are several possible ways to evaluate the relationships between State A, Group X, and Group Y.

Approach 1 – State A should never treat Group X and Group Y as participating in single armed conflict. Instead, State A should treat its fights with Group X and Group Y as two distinct NIACs.

In his 2013 report, the U.N. Special Rapporteur on Extrajudicial, Summary or Arbitrary Executions stated, “The established legal position is that, where the individuals targeted are not part of the same command and control structures as the organized armed group or are not part of a single military hierarchical structure, they ought not to be regarded as part of the same group, even if there are close ties between the groups.” That is, the Special Rapporteur would treat State A as being in two distinct NIACs with two distinct NSAGs, where each situation independently must meet the intensity threshold. This approach seems problematic, because it is easy to imagine in a case in which Group X exercises significant influence over the operations of Group Y, even as the groups’ command and control structures remain distinct. In such a case, it seems strained to treat the operations against Group X and Group Y as two distinct NIACs.

Approaches 2 and 3: These two approaches analogize from the concept of co-belligerency, which originated in international armed conflicts. Approaches 2 and 3 (described below) take different positions on what that concept requires.

Indeed, as Rebecca Ingber explains, the concept is surprisingly ill-defined. The major division among commentators is whether Group Y must itself actually engage in hostilities against State A, or whether Group Y must only provide significant assistance to Group X in order to join the NIAC as a “co-belligerent” (or a functional equivalent). The U.S. government seems at different times to have adhered to both approaches 2 and 3. The ambiguity surrounding its approach stems from the use of the phrase “entered the fight,” which the United States uses to define what constitutes an “associated force” of al Qaeda or the Taliban.

Approach 2 – Group Y must actually fight against State A in order for Groups X and Y to be deemed functional co-belligerents.

This approach demands evidence of Group Y’s engagement along two axes: the axis between Group X and Group Y, and the axis between Group Y and State A. Morris Greenspan’s traditional discussion of co-belligerency described a co-belligerent as a “fully fledged belligerent fighting in association with one or more belligerent powers.” Greenspan thus would seem to require actual fighting between Group Y and State A. In a more modern discussion of whether co-belligerency is a suitable concept for NIACs, Nathalie Weizmann writes, “[F]or 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 [the original NSAG].” The U.S. district court assessing the habeas petition of Hamlily, a Guantanamo detainee, adopted Greenspan’s “fully fledged belligerent” language, noting that the concept of “associated forces” does not include “terrorist organizations who merely share an abstract philosophy or even a common purpose with al Qaeda – there must be an actual association in the current conflict with al Qaeda or the Taliban.” Each of these approaches seems to require engagement on both axes before Group Y could be deemed part of the State A/Group X NIAC.

At first glance, the U.S. government appears to employ this approach. In explaining how the United States interprets the idea of an “associated force” in U.S. domestic law, former DOD General Counsel Jeh Johnson noted that an associated force must be “(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.” This definition appears to indicate that the United States will only treat as a functional co-belligerent to al Qaeda those groups that have engaged in hostilities against the United States. As of spring 2015, the United States interpreted that test to be met by “insurgent groups in Afghanistan; al Qaeda in the Arabian Peninsula (AQAP) in Yemen; the Khorasan Group in Syria; and ISIL.” By December 2016, the United States had begun to treat al Shabaab as an associated force of al Qaeda as well.

Approach 3 – Assistance by Group Y to Group X in Group X’s NIAC against State A is enough to render Group Y a functional co-belligerent, even if Group Y does not directly engage in hostilities against State A.

In this approach, Group Y becomes a functional co-belligerent of Group X by providing substantial support to Group X of the kind that would historically have made a neutral state a co-belligerent of a state party to an international armed conflict (IAC). But Group Y need not directly engage in hostilities against State A; substantial assistance to Group X is enough to trigger Group Y’s functional co-belligerency.

A key source of this approach is Curt Bradley’s and Jack Goldsmith’s article exploring the 2001 congressional authorization for the use of force. In examining how close an affiliated terrorist group must be to al Qaeda to render that group part of the same “organization” as al Qaeda, the authors identify co-belligerency and the law of neutrality as the most relevant source of guidance. To avoid becoming a co-belligerent with a state in its IAC, a “neutral state must not participate in acts of war by the belligerent, must not supply war materials to a belligerent, and must not permit belligerents to use its territory to move troops or munitions, or to establish wartime communication channels.”

Adapting that doctrine to the non-state actor context, they write:

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.

Several of these examples do not entail hostilities between the United States and the terrorist organization assisting al Qaeda. Approach 3 thus sets a lower bar for a state to treat Group Y as a functional co-belligerent of Group X.

This approach may more accurately describe the U.S. approach in practice. Not only has the government cited the Goldsmith/Bradley article in its briefs, but one commentator also suggests that the Executive “may accept some level of support to al Qaeda . . . as sufficient to render a group a co-belligerent.” It is also possible that the United States is maintaining deliberate ambiguity or is internally divided about the best legal reading of co-belligerency rules.

Approach 4 – Use the ICRC’s “direct participation in hostilities” (“DPH”) factors to evaluate Group Y’s efforts in relation to the State A/Group X NIAC.

Although the question at hand is distinct from the question of whether a single person can be targeted because he is taking direct part in hostilities, there may be some utility in considering the DPH factors in evaluating associations between groups in their adverse relationship to the state. This is because the DPH factors are intended to assess the links between military-like acts by an actor who does not fall within the core of a NSAG, and an existing NIAC (or IAC) itself. That is similar to the inquiry undertaken here.

In short, the three-prong ICRC DPH test asks (1) about the nature of the hostile act; (2) about the level of causation between the act and the harm likely to result; and (3) whether the act was designed to cause harm in support of one party and to the detriment of the other. Translated into the functional co-belligerency inquiry, State A would ask:

Was Group Y’s participation or action likely to adversely affect the military options of State A?

Did Group Y’s participation directly lead to tangible harm to State A?

Was Group Y’s participation intended to benefit Group X and harm State A?

If the answer to each question is yes, we might conclude that State A may treat Group Y as a functional co-belligerent of Group X.

Conclusion

This post set forward several options for thinking about how to evaluate connections between NSAGs. Going forward, on what basis should states choose among approaches to this issue? Does it make sense to try to transpose the law of co-belligerency to NIACs, or does the nature of NSAGs render the analogy too strained? Does an analogy to “direct participation in hostilities” make more sense, even though that concept was developed to assess individual targetability? At the end of the day, states may conclude that the choice among approaches rests as much on policy and practical considerations as on legal ones.

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