The present symposium follows on from a forum held at the University of Oslo on 11 September 2015 during the 11th Annual Conference of the European Society of International Law. This forum, sponsored by the ESIL Interest Group on Peace and Security, was particularly successful and led to a very interesting debate between the members of the European Society of International Law. The Leiden Journal of International Law has today the pleasure to publish, after the usual process of peer review, the complete and updated versions of the four papers presented during this event.

The legal argument of intervention by invitation has been used by all 11 states intervening in Iraq (nine members of the US-led coalition, Russia, and Iran), by Egypt for its airstrikes against ISIL in Libya, and by Iran and Russia for their interventions in Syria. To the extent that these consensual military interventions targeted ISIL and other UN-designated terrorist groups, their legality has not been challenged by any state. Strong criticisms marked nonetheless military operations undertaken by invitation, but not targeting ‘terrorist groups’, such as the alleged Russian strikes against the ‘Syrian moderate opposition’. The arguments advanced by intervening states seem to confirm the purpose-based approach of intervention by invitation. They did not claim a ‘right to intervene in a civil war’, but relied on the existence of both a request by the government and a legitimate purpose: fighting ISIL and other terrorist groups. The different reactions also show unwillingness by the international community to recognize a general right of intervention in a civil war. Turning to the strikes of the US-led coalition in Syria, it is impossible to rely on the legal basis of ‘intervention by invitation’. The controversial theory of ‘passive consent’ could be used to some extent – but not after September 2015 when Syria denounced these strikes as a ‘flagrant violation’ of its sovereignty. The current efforts of the international community to find consensual solutions to the dramatic conflicts in Syria, Libya, and Iraq could offer new possibilities of consensual interventions against ISIL and other terrorist groups.

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.

This article examines the law of self-defence as applied to non-state attacks in light of the coalition air strikes against ISIL in Syria. It critiques the two current interpretations of the law of self-defence – one based on attribution and the other on the ‘unable or unwilling’ test – for failing to address adequately the security threat posed by non-state actors or for not addressing convincingly the legal issues arising from the fact that the self-defence action unfolds on the territory of another state. For this reason, it proposes an alternative framework which combines the primary rule of self-defence to justify the use of defensive force against non-state actors, with the secondary rule of self-defence to excuse the incidental breach of the territorial state's sovereignty.

This article examines several questions relating to international humanitarian law (jus in bello) with respect to the conflicts against the Islamic State. The first question explored is the classification of conflicts against the Islamic State and the relevant applicable law. The situation in Iraq is a rather classic non-international armed conflict between a state and a non-state actor with third states intervening alongside governmental forces. The situation in Syria is more controversial, especially with respect to the coalition's airstrikes against the Islamic State on Syrian territory. If the Syrian government is considered as not having consented to the coalition's operations, then, according to this author's view, the coalition is involved in two distinct armed conflicts: an international armed conflict with the Syrian government and a non-international armed conflict with the Islamic State. The second question analyzed in the article bears on the geographical scope of application of international humanitarian law. In this context, the article examines whether humanitarian law applies: in the entire territory of the state in whose territory the hostilities take place, in the territories of the intervening states, and in the territory of a third state.