As readers of Lawfare know, a growing number of States believe that use of force in self-defense against a non-state actor on the territory of a third State, without the consent of that third State, may be lawful under international law if the non-state actor has undertaken an armed attack against the State and the third State is itself unwilling or unable to address the threat posed by the non-state actor. The content of the “unwilling or unable” test, its pedigree, and whether it has become a part of customary international law have been widely debated among international law scholars and practitioners, and one of us has addressed those issues extensively elsewhere.

Elena Chachko & Ashley S. Deeks, Who Is on Board with "Unwilling or Unable"?, Lawfare (October 10, 2016).