After establishing key trends in the evolution over centuries of the law relating to conflict, and having put forward a convincing methodology, the attempt to give meaning to the notion of the internationalisation of a conflict is the task of the first substantive chapter. Numerous ways in which various kinds of actor become involved in, or are affected by, an armed conflict are considered and are measured against the notion of internationalisation achieved in chapter 1 (p. 27). Consensual and non-consensual intervention are addressed and the idea that intervention by a state on the side of one of the warring factions in a failed state internationalises the conflict is acknowledged to be controversial (pp. 38-39). Presumably, if the circumstances giving rise to the absence of a functioning government also result in the absence of an essential element of statehood, it would be difficult to classify the pre-existing situation as a non-international armed conflict (NIAC) coming within common article 3, raising the spectre of a situation to which no international law applies.

The author rightly, in the present reviewer’s opinion, de-couples the law of state responsibility from conflict qualification, and brings the focus back to whether a state has used force through a non-state group (p. 44). Participation by the state in the organisation and coordination of the rebels is rightly seen as critical. The potential for international organisations to internationalise a conflict is noted as are the circumstances that risk dissolution and disintegration of states. Article 1(4) gets a proper mention, as do belligerency recognition, agreements under common article 3 and unilateral declarations by the territorial state. Whether the Israeli blockade of Gaza constituted belligerency recognition can be debated. Likewise, not every observer will agree with the author’s assertion that unilateral declarations by armed groups that they will apply certain rules in armed conflict are binding. To the extent that they reflect customary law, the relevant rules will of course be binding on that basis. The interesting question is whether non-customary rules will be binding by virtue of the declaration, and the present reviewer has his doubts (p. 82). The suggestion that such declarations can constitute relative internationalisation must be right, depending of course on what is being declared. Likewise, the conclusion that the parties may not ‘contract out’ of the application of IHL rules is widely accepted.

Where more than two contesting parties are involved in a complex conflict, some argue that external intervention renders the multiplicity of internal conflicts international in character while others argue that each conflict retains its own character notwithstanding the intervention. The author argues for a hybrid approach. Situations where “the mutual effect of the conflict pairs is insufficient to disrupt the parties’ autonomy should be seen as instances of separate uses of force, and thus as separate armed conflicts. But once the autonomy of the aligned conflict parties has been sacrificed, their use of force against the common enemy should be seen as a joint effort” causing all three parties to be in a single IAC. (pp. 103-104) So it is when the use of force by individual internal and external parties is indistinguishable that IAC application would be triggered.

The author correctly identifies IACs and NIACs as distinct legal notions and rightly centres the discussion of de-internationalisation on the transformation of a conflict involving the same parties from an IAC to a NIAC and, equally properly, requires a conflict emerging from such a de-internationalisation process to meet the criteria of organisation and intensity if it is to be classified as a NIAC. Where there are on-going hostilities between two states, unification and de-internationalisation cannot occur. It must be right that a withdrawal of a recognition of belligerency would have a de-internationalising effect, and that following a special agreement to apply IAC rights a subsequent agreement to re-qualify the conflict as a NIAC would have no legal effect.

In chapters 5, 6 and 7 the case is made that most fighters belonging to most non-state armed groups in most kinds of internationalised armed conflicts should be eligible for combatant status. The history reflected in chapter 5 reinforces the view that combatancy remains one of the features that distinguish IACs from NIACs, although the concept has been applied to a limited set of non-state actors.

Extending combatancy to fighters in internationalised armed conflicts would, it is powerfully and persuasively argued, incentivise compliance with distinction while being coherent with the equal treatment principle.

Then, on page 145, the author deals with what is frequently referred to as the combatant’s privilege. He submits “that it is imperative to understand the status as only a bar to prosecution”, and bases this on “profound moral and practical reasons” (p. 145). Article 43 API’s reference to a right to participate is argued away as a device to reach agreement on the combatant definition. It is therefore asserted that IHL does not create a ‘license to kill’. This reviewer, while not using the phrase, would respectfully take issue with the underlying position. The reference in article 43 to the right to participate arguably has its conventional law root in article 1 of the Hague Regulations and the reference to the ‘laws, rights and duties of war’. It is at least very arguable, and in this reviewer’s opinion the preferred view, that the law does not merely grant an immunity from prosecution for combatants. That would imply that a lawful act of war by a combatant remains a crime the prosecution of which is prevented by IHL. The better view, it is suggested, is that such acts that comply with IHL are deprived of their criminal character by the combatant’s privilege.

The analysis of the objections to the extension of combatant status to NIACs properly centres on the sovereignty issue, rightly seen as biting hardest in the non-internationalised context. Where fighters in internationalised armed conflicts are ‘subject to a hierarchical structure and strict enforcement mechanisms approximating those of full-fledged states’, one can agree with the author that prosecuting such individuals after the conflict would be unfair, because of the broad equivalence of the non-state actor with a state (p. 156) and would likely be counter-productive as potentially eroding IHL compliance (p. 158). The contention that the prosecution objection does not suffice as a basis for wholesale denial of combatant status is compelling, particularly given that resort to such prosecutions is ‘much less frequent [...] than one might expect’. (p. 163).

The author convincingly argues that combatancy under GC III applies to armed forces and is not dependent on their compliance with the four conditions. As to irregular armed forces, he notes that the ‘belonging to a party to the armed conflict’ stipulation raises some issues in relation to internationalisation. Observing the more relaxed regime under API, articles 43 and 44, the ‘command responsible’ language is rightly seen as posing a potential issue in conflicts internationalised by intervention, but article 44(6) probably does ‘save the day’.

The book then addresses occupation, a situation that marks out IACs from NIACs. The book argues that in principle occupation should be seen as applicable to internationalised armed conflicts, recognising of course the academic and practical controversies and complications. Chapter 8 traces the history of evolving occupation law noting its roots in inter-state situations and in what is described as the “individualization of the law of belligerent occupation” (p. 195) i.e. bringing individuals’ interests to the fore. Objections to the practical applicability of occupation law to internationalised armed conflicts based on perceived lack of legal personality of armed groups, on issues of sovereignty and on their suggested inability to apply such law are considered in turn. The analysis, perhaps controversial to some and de lege ferenda to others, is compelling, detailed and cites practical examples that merit careful consideration. Also controversial and maybe also de lege ferenda are the views: that international law foresees that a state can occupy its own territory; that such occupation can relate to territory previously held by a non-state group; and that a non-state group can occupy a state’s territory (p. 220). The present reviewer tends to favour the Baxter approach, cited on page 219, that “[t]erritory cannot be belligerently occupied by the lawful government or the rebels”. However, with the increasing frequency of NIACs and their internationalisation, perhaps prevailing legal opinion will either regard belligerent occupation as applicable in such circumstances, or will at least require that relevant rules based on occupation law be applied by analogy. Chapter 10 proposes criteria to determine the scope of application, but the reader may feel that while it may indeed be possible to rationalise the rules of occupation law, or most of them, with the circumstances of internationalised armed conflicts, it will only be if states generally do so that the relevant rules will actually apply. It will be for the reader to review the evidence cited in support of the argument, evidence which is clearly the product of impressive and meticulous research, in order to form his or her own conclusions.

This is a carefully written, well-presented and logically argued analysis of an important yet under-discussed topic. It explains the law with admirable authority and charts the way forward for both academia and policy makers. It is a very fine piece of work.