69 Pages Posted: 12 Dec 2011 Last revised: 3 Apr 2012

Date Written: August 19, 2011

Abstract

Non-state actors, including terrorist groups, regularly launch attacks against states, often from external bases. When a victim state seeks to respond with force to those attacks, it must decide whether to use force on the territory of another state with which it may not be in conflict. International law traditionally requires the victim state to assess whether the territorial state is “unwilling or unable” to suppress the threat itself. Only if the territorial state is unwilling or unable to do so may the victim state lawfully use force. Yet there has been virtually no discussion, either by states or scholars, of what that test requires. The test's lack of content undercuts its legitimacy and suggests that it is not currently imposing effective limits on the use of force by states at a time when trans-national armed violence is pervasive.

This Article provides the first sustained descriptive and normative analysis of the test. Descriptively, it explains how the “unwilling or unable” test arises in international law as part of a state's inquiry into whether it is necessary to use force in response to an armed attack. It identifies the test's deep roots in neutrality law, while simultaneously illustrating the lack of guidance about what inquiries a victim state must undertake when assessing whether another state is “unwilling or unable” to address a particular threat. Normatively, the Article plumbs two centuries of state practice to propose a core set of substantive and procedural factors that should inform the “unwilling or unable” inquiry. It then applies those factors to a real-world example – Colombia's use of force in Ecuador in 2008 against the Revolutionary Armed Forces of Colombia – to explore how the use of these factors would affect the involved states' decision-making and the evaluation by other states of the action's legality. The Article argues that the use of these factors would improve the quality of state decision-making surrounding the use of force in important substantive and procedural ways.