Michael N. Schmitt*
In October 2014, the United States and various other States commenced an air campaign against the “Islamic State” in Iraq and Syria and the Khorasan Group in Syria. The international law governing when a State may use force against non-State actors on the territory of another is generally unsettled, except when either the territorial State has granted its consent to the operations and those operations remain within the scope of the consent or the United Nations Security Council has authorized them pursuant to its U.N. Charter, Chapter VII, authority. Beyond those justifications, States remain divided as to how to lawfully respond to armed non-state actors acting independently of a State without violating the use of force prohibition in Article 2(4) of the U.N. Charter, as well as the sovereignty of the territorial State. In particular, the nature of the right to use force in self- defense in such circumstances remains hotly contested.
The current military operations against the “Islamic State” and the Khorasan Group have each been justified on different legal grounds. This article critically evaluates the legality of the respective operations to isolate the legal questions that continue to divide opinion on the use of force by States against non-State armed groups located in other States.
* Honorary Senior Research Fellow, Exeter University Law School; Associate Fellow, Chatham House.
* Charles H. Stockton Professor and Director, Stockton Center, US Naval War College; Professor of Public International Law, Exeter University; Fellow, Program in International Law and Armed Conflict, Harvard Law School. The views expressed in this article are those of the authors in their personal capacity.