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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.

Publication Date





Yale Journal of International Law

First Page



International Law