The Limits of Self-Defense Against Non-State Actors: The Continuing Duty to Meet the ‘Unable or Unwilling’ Jus Ad Bellum Test During Hostilities

Posted: 5 Mar 2018

See all articles by Tess Bridgeman

Tess Bridgeman

Stanford University - Center for International Security and Cooperation

Date Written: February 24, 2018


Under international law, a state may use force in self-defense when subject to an armed attack or an imminent threat thereof, so long as the defensive use of force meets the necessity, proportionality, and immediacy requirements of the jus ad bellum - the law governing the resort to force. These core requirements address whether resort to force is permissible, but not where a use of force in self-defense may occur. In the context of armed attacks by non-state actors occurring from the territory of a state (the "territorial state") other than the state that was subject to armed attack or imminent threat thereof (the "victim state"), the question of where force may be used can be particularly vexing. In recent decades, several states have relied on a controversial theory that permits a victim state to use force in self-defense against non-state actors in a territorial state without that state's consent, so long as the victim state determines that the territorial state is "unable or unwilling" to effectively address the threat posed by the non-state actors. Scholarship has begun to flesh out the test for invoking this self-defense theory as an element of the necessity prong of the jus ad bellum. However, its application over time following a state's initial jus ad bellum analysis, and the doctrine's outer limits more broadly, remain largely unexplained by states or scholars.

Recent state practice makes clear the need to elucidate limiting principles following the invocation of the "unable or unwilling" theory at the initial resort to force. For example, the United States' use of force against the Islamic State of Iraq and the Levant (ISIL) and al-Qa'ida (AQ) in Syria that began in 2014 without Syrian consent has been justified on this basis. The operations of the United States and its partners in Syria raise serious questions about the limits of this theory, which has been claimed to provide a legal basis for an ever-broader array of military objectives, even while the capabilities of the territorial state have increased and those of the aggressor non-state actors have declined.

This article seeks to put forward a more comprehensive understanding of the "unable or unwilling" doctrine that examines not just the test for its initial invocation, but also its contours and application over time. First, after briefly surveying the current understanding of the "unable or unwilling" test among states and in the academic literature, this article explains that a state has a continuing duty to meet the jus ad bellum requirements of the "unable or unwilling" test throughout its use of force justified on that basis. Just as in a consent-based scenario, where the lawfulness of the exercise of self-defense depends on certain requirements being met in the first instance and the parameters of genuinely provided consent not being exceeded during the use of force within the territorial state, a victim state operating on an "unable or unwilling" basis also has a continuing obligation to ensure the requirements of the "unable or unwilling" test are met at all times subsequent to the initial resort to force.

Second, this article explores the factors that must be considered in determining whether the "unable or unwilling" standard is met in its jus ad bellum analysis prior to the initial resort to force - which this article will argue sound in both necessity and proportionality - and examines how those factors can be applied after hostilities are underway. Finally, this article puts forward a non-exhaustive set of conditions that should prompt a victim state to examine whether it continues to meet the "unable or unwilling" standard or whether changed circumstances necessitate the cessation of its military operations within the territorial state.

Keywords: unwilling, unable, self-defense, jus ad bellum, non-state actor, necessity, proportionality, use of force, consent

Suggested Citation

Bridgeman, Tess, The Limits of Self-Defense Against Non-State Actors: The Continuing Duty to Meet the ‘Unable or Unwilling’ Jus Ad Bellum Test During Hostilities (February 24, 2018). Available at SSRN:

Tess Bridgeman (Contact Author)

Stanford University - Center for International Security and Cooperation ( email )

United States

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