The ‘Unwilling or Unable’ Test: Has it Been, and Could it be, Accepted?

Abstract On 23 September 2014, the United States of America sent a letter to the Security Council justifying the launch of an air campaign against the Islamic State of Iraq and the Levant (ISIL) on Syrian territory. In this letter, the US referred to a formula that appeared a few years ago in certain scholarly writings: the ‘unwilling or unable’ test. The aim of this article is to show that this test has not been accepted by the international community of states as a whole in the Syrian case. It is also to stress that such an acceptance would lead to a radical transformation of the jus contra bellum regime, one that a large majority of states is probably not ready to accept.