From South Asia to Europe and beyond, government lawyers have to inquire as to when a State is legally justified in resorting to military force against a non-State actor. What body of law do they apply in answering these queries? Unfortunately some discussions in this legal space risk applying inapplicable bodies of law due to conflation with the more familiar legal regime governing resort to force between States, and we hope in the following analysis to provide some clarity.

An armed attack by a non-State actor (including organized armed groups and individuals) can trigger a State’s right to exercise self-defense, which affects the prerogative of that State to use force inside other countries. That said, non-State actors do not possess legal rights like States do and they are not the relevant subjects of the so-called jus ad bellum—a set of rules contained in the UN Charter and customary international law exclusively regulating the use of force between States.  Thus, Article 2(4) of the UN Charter provides that “All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state….”

Accordingly, questions such as whether a non-State armed organization must pose an imminent threat before a State can use force against the group under jus ad bellum seem conceptually misplaced. To put the point more bluntly, that line of reasoning involves a category mistake. We offer the following thoughts as a corrective.

I. Framing the legal question properly

Many international lawyers conflate the jus ad bellum rules that determine the legality of a State’s use of force against another State (or on the territory of another State without consent of the territorial Government) with rules that might apply to whether a State can use force against a non-State actor.  These two categories of action—use of force against the sovereign interests of another State and use of force against a non-State actor—must be understood in their own legal terms.  Jus ad bellum rules developed in a State-centric system, in which the relationship between States dominated the international legal framework.  Meanwhile, international law was not designed to speak directly to State’s rights to use force against non-state armed groups or individuals—particularly in an acting State’s territory, with the consent of another sovereign, or beyond any State’s territory (e.g., in “international waters”).

To help specify the proper legal question, then, consider a few scenarios in which an element of the equation—non-consensual use of force against another State—is simply not at issue:

  1. State A employs force against a non-State actor inside State A’s own territory
  2. State A employs force against a non-State actor inside State B’s territory with State B’s consent
  3. State A employs force against a non-State actor beyond any State’s territory (e.g., beyond the territorial seas of any coastal state/in “international waters”)
  4. State A employs force against a non-State actor inside State B’s territory without State B’s consent on the basis of State A’s inherent right to self-defense.

The 4th scenario has already been addressed at some length by the U.S. Government (see p. 10) and by a former principal Legal Adviser of the UK Foreign & Commonwealth Office.  Both accept that a State may take armed action against a non-State actor on the territory of another State, without the consent of the territorial Government, if the territorial government is unwilling or unable to effectively restrain the armed activities of the non-State actor such as to leave the state that has a necessity to act in self-defense with no other reasonably available effective means to address an imminent or actual armed attack.  We bracket that example here for sake of focusing on the use of force against a non-State actor in which the interests of all affected States are aligned against the non-State actor.

In short, the properly specified question: What law applies to State A’s decision to resort to force against the non-State actor in scenarios 1-3?

II. The wrong answer

We have already outlined the jus ad bellum constraints embodied in the UN Charter, which do not regulate a State’s use of armed force against a non-State actor.  [There may be rules relevant to a State’s use of force that exist in customary international law but not in the UN Charter. We acknowledge that point, but treat the UN Charter and customary rules as coterminous here.]

A flurry of recent writings by international law experts acknowledges that jus ad bellum does not address the question we have specified. At Just Security, these writings include:

Eliav Lieblich’s post – “What Law Applies to the Resort to Force Against Non-State Actors? Filling the Void of “Internal Jus Ad Bellum” – which builds on his law review article;
Jan Hessbruegge’s post – “The Right to Life as the Jus ad Bellum of Non-International Armed Conflict (A Reply to Lieblich)” – which builds on his forthcoming book; Anthony Dworkin’s post – “Human Rights Law is the Legal Basis for Use of Force Against Non-State Armed Groups—But What Follows?” – which presages his forthcoming law review article

This is not a new revelation. In an exchange among four scholars back in 2010, Marko Milanovic over at EJIL Talk! noted their “broad agreement that the prohibition on the use of force in international law, as set out in Article 2(4) of the UN Charter, operates exclusively between states.”

That said, we do not mean to suggest that the right of self-defense, under Article 51 of the UN Charter and custom, is always irrelevant in armed interactions between a State and a non-State actor. As we mentioned at the outset, for instance, a State’s right of self-defense under the Charter can be triggered by an armed attack or threat of armed attack from a non-State actor, which would affect the prerogative of that State to use force inside other countries.

III. What fills the Gap?

We choose not to give our own definitive answer here to the question of what fills the gap. We may even disagree between ourselves. That said, certain threshold questions do not seem insurmountable as they have been the subject of much scholarship and a fair amount of contemporary state practice.  When may a State deviate from a law enforcement paradigm to an armed conflict paradigm?  May a State consent to another State using force against a non-State actor within the consenting State’s territory?  If so, what, if any, legal restrictions apply to the operations–e.g., do legal obligations such as human rights treaties to which the consenting State is a party apply to the acting State?  What obligations impose constraints on the use of force outside State-controlled territory, such as in “international waters”?  And once those questions have been answered satisfactorily in regard to particular facts and circumstances at hand, then States may find themselves in an environment in which there are sound legal justifications for resorting to force against an non-State actor, subject to compliance with other traditional international law principles (e.g., compliance with jus in bello when conducting hostilities in armed conflict, gaining consent of the sovereign where required).

A couple notes are worth making at this point specific to human rights law. Some of the authors we mention above suggest that international human rights law fills this void.  With that in mind, it is important to note that some States, even if they accept that human rights law applies extraterritorially, for example, as a matter of a regional human rights treaty that binds them (the ECHR) or as a matter of custom, may still not consider that law applicable to situations in which they lack effective control (i.e., air campaigns involving lethal targeting). That said, this is a complicated area of law that would need to be sorted out. For example, what if a State captures members of a non-State armed group outside of its own territory? Perhaps the State’s custody over those individuals would satisfy the relevant part of the jurisdictional analysis (the effective control test) under treaty-based or customary law.

While we do not aim to resolve those kinds of questions, we do note some implications that flow from our analysis, that is, from the basic insight that the jus ad bellum rules do not apply.

IV. Implications

Although we do not answer the question set forth in Part III, some important implications can be drawn from our analysis.

1. Our observation that the established rules of jus ad bellum do not regulate States’ rights to use force against non-State armed groups when the sovereign interests of another State are not implicated could also support a conclusion that jus ad bellum likewise does not regulate when non-State actors may resort to force against a State. That is, we admit that States being unbound by jus ad bellum in their resort to force vis non-State actors may have a reciprocal implication for non-State actors in theory.

That said, in practice, the use of force against a State by a non-State actor will almost always be subject to punishment under the domestic law of the relevant State, because, outside the perhaps antiquated practice of “recognition of belligerency,” non-State actors do not enjoy “combatant immunity” for their belligerent acts.  Nonetheless, we acknowledge and agree that conduct by non-State actors is not regulated by established jus ad bellum. (Note, this point has potential implications for a range of other issues including remote cyber operations by non-State actor hackers against government facilities in a State’s sovereign territory.)

2-a Our analysis suggests less reason to worry that a State’s broadening the scope of its prerogatives to use force against non-State actors would, or analytically should, spill over into States’ prerogative to use force against another State.  So long as practitioners recognize that the bodies of law are distinct, state practice in resorting to armed force against non-State actors does not form precedent relevant to the lawfulness of a resort to force against another State.

2-b At the same time, our analysis also allows States to broaden their conception of their prerogatives in the context of responding with force to non-State actor terrorist threats, without having to justify such a decision according to long-standing jus ad bellum rules (as long as we are limited to scenarios 1-3 as outlined in the first Part).

In short, for both 2-a and 2-b, one of the most important lessons to be drawn from our analysis, is that these are two separate legal regimes: one involving a body of international law that “operates exclusively between states,” as Milanovic put it, and the other involving the potential legal regulation of State’s use of force against a non-State actor. Accordingly, opinio juris such as a statement by a senior Government official to the effect that applying a “more flexible understanding of ‘imminence’ may be appropriate when dealing with terrorist groups” should not bear on the analysis of “imminence” when contemplating a use of force between States in future conflicts. This is not only true as a formal matter, but also as functional and normative one. There may a host of different reasons for preserving State cooperation and balancing other interests when developing and interpreting rules that regulate the use of force between States, which are not easily transposed onto relations between States and non-State actors.

3. Finally, our analysis helps to illuminate a form of analytic bootstrapping that some use (often unintentionally) in arguing that once a State enters into an “armed conflict” with a non-State actor, the State then has the legal right to resort to force against the non-State actor. If jus ad bellum is not the applicable body of law regulating the resort to force against a non-State actor, neither is jus in bello. Hessbruegge makes the point when he writes: “This raises the conundrum whether a government can simply escalate an armed confrontation with violent non-State actors from law enforcement action to a non-international armed conflict with the result that its security forces can then rely on the more permissive hostilities paradigm to use force. To say the least, this would entail a perverse incentive: the more violence a government de facto uses, the more violence it is de jure entitled to use.”

What then are the justifications required for a State to resort to force against a non-State actor? We know where not to find the answer to that question.


As with our previous post, we thank once again our former colleague Michael d’Annunzio for significant inspiration in helping us formulate the ideas in this analysis. The views expressed herein are those of the authors and do not necessarily represent the views of the Department of the Navy, the Department of Defense, or the United States.