James A. Green's monograph offers a timely, extensive, and scholarly account of a crucial but under-theorized area of jus ad bellum. As Green notes, collective self-defence (CSD) has become an increasingly invoked legal justification for the use of force, especially over the past decade (p. 1). This 388-page book is one of the most detailed and extensive examinations devoted specifically to CSD, providing a thorough doctrinal account firmly rooted in state practice and an advanced understanding of this pivotal modern justification for interstate force.

The book commences by evaluating various theoretical conceptions of CSD advanced in scholarship and judiciously concludes that the right is best understood as “the defence of another” state (p. 45). This conflicts with theories insisting that co-defending states demonstrate self-interest. Green incisively dismantles the latter theories due to meagre foundations in customary law (pp. 25–46). He then establishes that CSD has become conjoined with individual self-defence in the UN Charter, irrespective of the limited pre-1945 resemblance beyond common treaty relationships (Chapter 2). After charting its historical development in the first two chapters, Green methodically analyses the criteria for its lawful exercise – both those it shares with individual self-defence (Chapter 3) and those unique to it, chiefly the requirement for a valid request by the state seeking assistance (Chapters 4–6). The extensive Chapter 7 examines the nature and role of CSD alliances. The book concludes by exploring the relationship between CSD and the closely related, but distinct, concept of military assistance on request.

A major strength of the book lies in its rigorous examination of state practice. With CSD being largely regulated by customary international law, Green rightly focuses on how states have invoked and responded to invocations of this right since the establishment of the UN. Through this analysis, he makes a persuasive case that while a request for assistance by the defending state is an established legal requirement, no separate formal declaration of being under attack is required (p. 167) – an important clarification of the law.

Equally valuable is the book's extensive discussion of factors affecting the validity of such requests (Chapters 5–6). Questions such as who exactly can issue a request, in what form, to whom, and when, have practical implications but receive little academic attention. By tackling these issues head-on by assessing state responses to controversial requests, Green makes an original contribution. He cautiously concludes that while statehood and, perhaps, UN membership is required of the requester, more subjective political considerations influence state assessments of validity in context-specific ways.

Inevitably for a book of this scope, limitations exist, especially given recent developments in Ukraine. Furthermore, while Green analyses the statehood requirement for requesting aid, questions about subsidiary organs administering territory remain. One could also critique Green's doctrinal approach as overly technical, underemphasizing policy implications, such as those regarding weapons transfers. Additionally, further analysis of issues like CSD's relationship with UN Security Council authorizations could provide useful context.

Overall, Green advances academic and policy debates on CSD. By clarifying the law and exposing areas requiring further study, it makes a timely and original contribution. The extensive examination of state practice ensures it will be an invaluable and essential reference for scholars, practitioners, and courts.