Introduction

Since the end of the Cold War, jus ad bellum debates have focused by and large on two main bones of contention. First, the vexing issue of the permissibility of unilateral humanitarian intervention has surfaced repeatedly in the wake of humanitarian crises in Kosovo, Syria and elsewhere. Second, with the advance of transnational terrorist groups such as Al Qaeda and ‘Islamic State’ – and, to lesser extent, the proliferation of weapons of mass destruction – the outer boundaries of the right of self-defence have been the subject of ample discussion. The legality of self-defence against attacks by non-state actors in particular has attracted enormous attention among states and international lawyers in the post 9/11 era. By contrast, far less attention has been paid to so-called ‘military assistance on request’ or – as some would have it ‘intervention by invitation’ (even if the idea of a consensual ‘intervention’ is, strictly speaking, a contradictio in terminis). It is striking, for instance, that Louise Doswald-Beck’s seminal article in the 1985 British Yearbook of International Law remains perhaps the most well-known treatise on the topic. Recent years have nonetheless seen a striking resurgence of – at times highly problematic – cases of ‘military assistance on request’, raising important questions about the legal parameters of this justification for the use of force. Without claiming exhaustivity, recent cross-border military operations that have been justified on the basis of consent include the Saudi-led operation Decisive Storm in Yemen to support President Hadi against the Houthi rebels (2015-ongoing); the Russian intervention in Syria (2015-ongoing) pursuant to a request from the Assad regime; the actions of the US-led coalition against the Islamic State (Operation Inherent Resolve) on Iraqi soil on the invitation of the Iraqi authorities (2014-ongoing); the Kenyan intervention in Somalia (inter alia in the context of Operation Linda Nchi (2011)); France’s Opération Serval in Mali (2012-ongoing (nowOpération Barkhane)); the 2017 ECOWAS intervention in The Gambia, at the request of President-elect Adama Barrow; the Russian intervention in Crimea (2014), justified in part by reference to an