The article provides a critical overview of the history, theoretical foundations and rules and principles governing the use of reprisals in international law. One means by which states can enforce international law is through the use of reprisals or countermeasures. In the interwar era, international lawyers sought to design a legal apparatus aimed at governing the use of reprisals in the international society. They recognized that violence could constitute a legitimate source of authority and justice in the international legal system. The post-1945 system of international law incorporated this idea in an attenuated form. This article sheds some light on a number of intricacies that international lawyers have historically had trouble dealing with in the pre-United Nations Charter era and that the contemprary system of coutermeasures seems to ignore, namely the paradoxical position that violence occupies as a source of authority and justice in international law : violence is both necessary and impossible in the international legal system.