Rebecca Ingber on Co-Belligerency in International Law and Under the AUMF

By Kenneth Anderson
Wednesday, September 21, 2016, 12:53 PM

Rebecca Ingber (international law professor at Boston University and Lawfare contributor) has posted a new draft paper to SSRN (forthcoming in 42 Yale Journal of International Law No. 1, 2017), titled “Co-Belligerency.” It’s an excellent and thoughtful article particularly relevant to the 15th anniversary of 9/11. Its topic, as the title indicates, is co-belligerency, in the context of analyzing what groups the U.S. government designates as a targetable co-belligerents, considered under both international law of co-belligerency and U.S. domestic law—and particularly the 2001 AUMF.

More exactly, Ingber argues that the U.S. government, under the Obama and Bush administrations, has long relied upon a doctrine of international law, co-belligerency, in order to give support to its domestic law designations of various groups under the AUMF—but that the international law doctrine does not give anywhere near the support for the domestic law interpretations and designations as the U.S. government’s statements over the years would appear to suggest. The article walks through both international law concepts of co-belligerency and U.S. domestic law, especially constitutional law doctrines of the president’s powers, in the process of reaching its conclusions about what it finds to be the overreach of successive presidents through the rubric of the AUMF.

This is a very interesting, well-researched and argued article—and one that bears reading by those in successive U.S. administrations seeking to both justify and cabin the scope of the AUMF. I would pose one question for Ingber—though it is one that reaches beyond the scope of her article and isn’t something that I’d expect to see addressed in the current piece. Lawfare posts by Bobby, the late and much-missed Michael Lewis, and others such as Ryan Goodman (as well as various scholarly articles, etc.) have talked about “co-belligerency” in what might be called the “flip-side” context—viz., what are the legal criteria for the U.S. to be engaged as a co-belligerent with some party to a conflict, and what are the legal consequences of that status.

Beyond the question posed by the article (what does co-belligerency mean as a legal designation applied to some non-state actor armed group or terrorist organization under the AUMF?), in other words, what are the legal criteria and consequences of “co-belligerency” where the U.S. is, explicitly or implicitly, engaged in use of force operations alongside a party to a conflict? Bobby, especially, has raised this issue in relation to an earlier phase of Yemen’s conflicts—e.g., when the US government was engaged in drone strikes and other activities in support of the Yemen government against groups that were engaged, internally, in civil war against the government and, externally, in supporting terrorist actions against the U.S. and others. Several commentators suggested that co-belligerency was the most accurate legal description of the U.S. role in those cases; there was debate over what that meant as a legal status if it were taken as applicable to the U.S.

This question, while important and interesting, is beyond the scope of Ingber’s current article, however. Still, it might be worth trying to see, in some future analysis, how the two—US government using “co-belligerent” as targeting authority and U.S. government as co-belligerent—might fit (or not) with each other. Meanwhile, Rebecca will be posting her own discussion of the article here on Lawfare; the article’s abstract from SSRN is provided below, but stay tuned for her own thoughts on the piece.


Executive branch officials rest the President’s authority in today’s war against ISIS, al Qaeda, and other terrorist groups on an expansive interpretation of a 15-year-old statute, the 2001 “Authorization for Use of Military Force” (AUMF), passed in the wake of the 9/11 attacks. They rely on that statute to justify force against groups neither referenced in—nor even in existence at the time of—the 2001 statute, by invoking a creative theory of international law they call “co-belligerency.” Under this theory, the President can read his AUMF authority flexibly, to justify force against not only those groups covered by the statute, but also new groups that “join the fight.”

In relying on “co-belligerency,” executive branch officials maintain that the President’s authority is bound by a clearly constraining rule with an established legal pedigree, but the co-belligerency theory does not in fact deliver on either. Instead, the Executive’s position is fluid, evolving, internally contested, and—contrary to the assurance that it has a firm foundation in international law—rests on shaky doctrinal grounds. In fact, the record suggests that executive branch officials are not even unified themselves on what the concept means or where it comes from. And yet the existence of this contested idea nevertheless acts as some impediment if not a barrier to executive action. It is, in effect, a grey-ish legal space, dangerously close to what David Dyzenhaus has called a “legal grey hole,” a mere “façade” of legal constraint. This article presents a story of a creative idea that became entrenched law, but in the process lost much of its shape. The result has been neither a clear limit on Presidential power, nor an executive branch run completely amok, but rather an amorphously-defined pool of discretionary authority for the President that few if any fully understand.