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.