Professor Christian Henderson's new textbook on the law on the use of force (jus ad bellum) is comprised of ten chapters (organized into four parts: the prohibition on the use of force; collective security; self-defence and; intervention in situations of civil unrest). Its key goal, as a textbook, is ‘to provide a contemporary introduction to international law governing the resort to force’ (3) for readers who are unfamiliar with that subfield. Yet, while the rules and debates set out in the book largely will be well known to ad bellum scholars, it nonetheless contains much that will be of value not just to beginners but those versed in the subject.

Providing meaningful depth, breadth and analytical heft while also ensuring clarity and accessibility is a challenge that many textbooks aspire to meet, not always successfully, and Henderson has—for the most part—achieved this balance extremely well. The book certainly covers all of the core aspects of the jus ad bellum, meaning that it acts as a ‘one-stop shop’ reference work. Indeed, it covers material that other leading textbooks on the law on use of force often do not, or, at least, do not in anywhere near the same amount of detail. Chapter 5's examination of United Nations peacekeeping, for example, expands the scope of the book beyond some traditional approaches to introducing the jus ad bellum.

There is impressive depth and analytical underpinning here too, especially for a textbook. To give just one example, Henderson grapples with complex and controversial discussions about the meaning of ‘imminence’ (297–307) to an extent and at a level that few targeted works on anticipatory self-defence manage. A key feature of the book is its extensive engagement with State practice and opinio juris throughout, which is, of course, crucial for understanding both the historical and recent development of the law on the use of force: ‘[l]ight is … shone upon these issues through an examination of … practice’ (159) (a statement made in the book specifically in relation to the Responsibility to Protect, but one that can been seen as nicely summarizing a core aspect of the overall methodology).

Despite the book's in-depth and analytical approach, care has been taken to maintain accessibility. Helpful questions and key pieces of reading are provided at the end of each chapter to support reflection and encourage further engagement. Similarly, the subsections in Chapter 1 (particularly at 31–40) covering various issues that relate to, but perhaps do not directly form an aspect of, the law on the use of force are indicative of the accessibility that the book aims for. These subsections are short and basic, and therefore may not be of much value to the average international lawyer, but they succinctly establish a wider context at the outset that will be invaluable for readers who are not just unfamiliar with the law on the use of force but international law generally.

There are, inevitably, aspects of the book that the present reviewer found less strong. Chapter 8 on self-defence against non-State actors, for example, arguably has problematic aspects. These include the apparent sympathy expressed by the author for a (highly disputable) distinction between responses against a State as opposed to those ‘purely’ against non-State actors within a State (eg 316, 321, 326), and a reiteration (eg 312–13) of a common (mis)reading of one aspect of the International Court of Justice's 1986 Nicaragua judgment (which only referred to the ‘effective control’ test in relation to the question of whether the US had violated the primary Article 2(4) norm; it did not indicate that the ‘substantial [State] involvement’ that the Court felt was required for an armed attack to have occurred necessarily needed to be premised on effective control).

It also is worth noting that, for all of the book's impressive coverage and depth, a few areas are given comparatively short shrift. It is something of a shame, for example, that force used for the protection of nationals (247–56) and collective self-defence (256–62) did not receive their own dedicated chapters, as might have been expected given the extensive coverage of most other areas. The so-called ‘accumulation of events’ theory, while perhaps not deserving of its own full chapter, also is dealt with in a manner that feels a little cursory (224–6, 239–40). At the same time, it is unclear why the book includes notable discussions of international human rights law and international humanitarian law in the section on targeted killings (338–44). These areas of international law are directly relevant to targeted killings, of course, but that equally is true for almost any use of force: the discussion—at least in this depth—of these other subfields here but not elsewhere feels anomalous, and a little unnecessary given the book's focus.

Such issues aside—they are minor, and no book is perfect—The Use of Force and International Law is, overall, an impressive work, which will challenge the established leading ad bellum textbooks—notably Christine Gray's International Law and the Use of Force and Yoram Dinstein's War, Aggression and Self-Defence (both of which recently have been updated: 4th edition, 2018, and 6th edition, 2017, respectively)—for the position of ‘go to’ introductory text/reference work on the law on the use of force. Expect it, in the same vein as those now classic books, to be updated in multiple editions. The Use of Force and International Law is a book that offers significant coverage of the topic without falling into the trap of being overly dense, meeting its primary goal of acting as an introduction for those who are new to the subject while also providing analysis at a level that will inform the work of established ad bellum scholars. It soon will be appearing on the reading lists for my students.