Collective self-defence can be defined as the use of military force by one or more states to aid another state that is an innocent victim of armed attack. However, it is a legal justification that is open to abuse and its exercise risks escalating conflict. Recent years have seen an unprecedented increase in the number of collective self-defence claims. It has been the main basis for US-led action in Syria (2014-) and was advanced by Russia in relation to its full-scale invasion of Ukraine (2022-). Yet there still has been little analysis of collective self-defence in international law. This book crucially progresses the debate on various fundamental and under-explored questions about the conceptual nature of collective self-defence and the requirements for its operation. Green provides the most detailed and extensive account of collective self-defence to date, at a time when it is being invoked more than ever before.