Belligerent reprisals consist in retaliatory non-compliance with the laws of armed conflict. A century-long reflection has not answered all the questions associated with their regulation, nor has the presence of new mechanisms to ensure compliance with the laws of war determined their dismissal. This dissertation imputes these ostensible paradoxes to a failure in determining the "causa" of belligerent reprisals; therefore, it sets out to investigate the purpose and function of such measures in the law governing international armed conflict. The first part of the thesis highlights the process by which belligerent reprisals have come to be construed as a tool to enforce ius in bello. It does so by unveiling how the notion has oscillated between two frameworks of reference, represented by peace-time countermeasures on one hand, and termination and suspension of obligations as a consequence of breach on the other. In the second part, that traditional understanding is challenged by an analysis of how belligerent reprisals are justified in the practice of States. In particular, a case-study of the arguments accompanying the “war of the cities” and chemical warfare during the Iran-Iraq War sheds light on the persistent influence of negative reciprocity in structuring the notion of belligerent reprisals. Relying on recent contributions from legal theory and political science, the thesis finally broaches a novel formalization of belligerent reprisals, re-interpreting them as a guarantee of symmetry in the legal position of belligerents and as a remedy against the disequilibrium between rights and obligations that is created by a breach of ius in bello.