Abstract
Countermeasures are established and instrumental aspects of the international legal system of self-help. Although they are of long lineage, countermeasures took center stage in international law discourse among States only with the advent of cyber operations as they appeared to offer injured States a legal basis for “hack backs.” This article examines the evolution of approaches to collective countermeasures initiated between States and reflected in the work of the International Court of Justice and the International Law Commission. Upon this groundwork, we survey and assess the international security conditions relevant to the issue of collective countermeasures, with particular emphasis on their use in cyberspace. We conclude that, though the issue remains unsettled as a matter of law, collective cyber countermeasures on behalf of injured States and support for the countermeasures of an injured State, are lawful.