Chapter 5 tackles the question of the applicability of belligerent reprisals in non-international armed conflict. After assessing the merits and difficulties associated with previous reflections on the topic, it devises a new methodology to approach the issue. Then, it puts the notion of belligerent reprisals in relation with the two features of inequality of status between States and non-State armed groups, and equality of rights and obligations for parties to non-international armed conflicts. A careful reading of the travaux préparatoires of Additional Protocol II to the 1949 Geneva Conventions upholds an interpretation that links belligerent reprisals with the latter principle, and that places reciprocity at the basis of both the applicability and the purpose of the measure in non-international armed conflicts. The chapter concludes with the impact of this formalization on such key questions as the requirement of imputability to a State of the original IHL violation and the actual features of the principle of equality. It suggests that the focus be shifted to the idea of equilibrium of rights and obligations, and that belligerent reprisals be seen as a key enabler of it.

Chapter 3 highlights several instances of State practice where the reciprocity paradigm continues to influence belligerent reprisals. Its bearing emerges from those formalizations of the mechanism that stress the purpose of restoring the balance in rights and obligations unduly disturbed by a breach of the laws of armed conflict. The chapter will first retrace this interest in several positions expressed by States before, during, and in the aftermath of the Geneva Diplomatic Conference that led to the adoption of the 1977 Additional Protocols to the 1949 Geneva Conventions. It will then focus on the provisions of military manuals, with a particular focus on US practice and the latest Department of Defense Law of War Manual. Finally, it will provide an extensive and, under many respects, unprecedented analysis of the Italian case-law on World War II atrocities: this judicial practice, which has been revived only recently, has brought to the fore several elements that are strongly associated with reciprocity. The chapter will thus highlight notable examples in which the reciprocity paradigm contributes to defining the purpose and function of belligerent reprisals.

In Chapter 4, the Iran–Iraq War (1980–88) serves as a case-study to test (and ultimately refute) a purely enforcement-based formalization of belligerent reprisals. In the field of chemical warfare, reciprocity and enforcement are shown to converge (rather than exclude each other) in the operationalization of belligerent reprisals. Reciprocity is seen as inspiring both the purposes associated with the measure (restoring the balance of rights and obligations and countering unlawful military advantage) and the specific traits that it would take (in-kind breach). In the "war of the cities", belligerents resorted to reprisals with purposes that cannot be encapsulated in the enforcement paradigm. These included the function of ensuring equality of opportunities (as a form of negative reciprocity) and that of strengthening, enacting and agreeing on new standards of conduct when the specific content of applicable rules was not clear or settled (as an aspect of positive reciprocity). As a result, belligerent reprisals appear as a highly flexible tool by which parties to an armed conflict bargain, approve or refuse, and police the concrete legal framework governing wartime interactions.

Chapter 1 places the institution of belligerent reprisals in relation with the two conceptual frameworks of reciprocity and enforcement. First, it sketches the trajectories by which international law has approached the phenomenon of belligerent reprisals, identifying extant prohibitions and clarifying the requirements for their lawful adoption. After recalling outstanding questions in the international regulation of the mechanism, it describes the two paradigms that legal theory could draw from to conceptualize belligerent reprisals. On the one hand stands reciprocity, as embodied chiefly in the termination or suspension of the operation of a treaty as a consequence of its breach; on the other, the paradigm of enforcement as manifested in countermeasures. Having described their main tenets, the chapter shows how these two blueprints, despite co-existing in the early theories on belligerent reprisals, have come to be seen as mutually exclusive, thereby offering two clearly distinct alternatives for the following formalization of the purpose and function of the mechanism.

Chapter 2 explains how belligerent reprisals have come to be interpreted as tools to induce compliance with the laws of armed conflict. It does so by highlighting three cumulative processes. First, it looks at the role that post–World War II tribunals, the ICTY and the ICRC have played in stressing the procedural elements of belligerent reprisals, emphasizing the highly formalized set of steps to be taken before the adoption of the measure while downplaying the retaliatory act itself. Then, it claims that the main thrust of this proceduralization lies in the creation of a regulatory framework that attributes a specific legal meaning to the retaliatory conduct and, by so doing, allows for an assimilation of belligerent reprisals with the notion of countermeasures. In turn, this analogy leads to the attribution to belligerent reprisals of a sanctioning character that protects the primary norm from the risk of persistent non-compliance. The outcome of these three processes is the attribution to belligerent reprisals of a chiefly coercive purpose, interested in inducing compliance with the laws of armed conflict and markedly influenced by the enforcement paradigm.

Chapter 6 inquires into the legality and purposes of belligerent reprisals in non-international armed conflict. At the outset, it delves into the travaux préparatoires of Additional Protocol II to the 1949 Geneva Conventions to overcome the paucity of black-letter provisions on belligerent reprisals in this type of conflicts and identify relevant practice indicating which reprisals are prohibited (and which are permissible). Then, it looks into the work of several fact-finding commissions, mandated investigations and expert bodies addressing situations of non-international armed conflict (including those in Myanmar, South Sudan, Yemen and Syria) to gauge their formalization of the mechanism. The re-instatement of reciprocity in the functioning of belligerent reprisals emerges clearly from the purpose of evening out the legal and substantive imbalance brought about by enemy breaches. This analysis results in a novel understanding of belligerent reprisals as a tool concerned with the overall equilibrium in the legal relationship between parties to the conflict and aimed at remedying their inequality of status.

The Introduction explains the relevance of a theoretical inquiry into the purpose and function of belligerent reprisals. It highlights several examples in recent practice where the vocabulary of belligerent reprisals has been harnessed by parties to an armed conflict, pointing to the continued relevance of the institution in contemporary warfare. At the same time, it outlines persisting difficulties in the terminology, regulation and governance of reprisals, and shows that they all derive from the failure by international legal theory to give a proper legal vest to the purpose and function of the mechanism. It points to fundamental fallacies both in how the question has been approached, and in how it has been answered. It proposes an alternative to existing accounts and outlines how it will be investigated in the book.

The Conclusion draws on the findings of the book to analyse the main implications of a reciprocity-based understanding of belligerent reprisals. First, it distinguishes this formalization of belligerent reprisals from earlier theories stressing the law-making function of the measure. Then, it accounts for the continued relevance of belligerent reprisals even at a time when mechanisms monitoring and enforcing compliance with the laws of armed conflict gain momentum. Finally, it explains how a reciprocity-based interpretation of belligerent reprisals would affect follow-up reform of the mechanism – be it in the sense of fine-tuning its regulation, or in the sense of disposing of it altogether.