Assessing the impact that security concerns should have on the operation of international law is tough. Sitting right at the edge of law and politics, it requires the identification of the limits of law in high-stakes moments of serious political and legal contestation. The nature of security compounds this difficulty: it is a protean concept (p. 1) that, in recent years, has expanded its scope in international relations from being focused on the physical protection of a state's territory and population to covering risks to the economy, food and energy resources, health, and the environment (p. 2). In the face of this expansion, how should international courts decide when a state's security interest justifies derogation from an international law obligation? How much deference should judges have for politicians claiming their decisions are necessary for security reasons?

The above book provides a rigorous doctrinal analysis of how the concept of security is used in international law, including customary law, treaties, and courts. Nasu is an outstanding guide: his comprehensive description, explanation, and analysis is notable for its clarity and accessibility and is an invaluable practical resource for understanding how security can and should be used in legal argumentation.

The book has an elegant structure. Chapter 1 begins by using security studies scholarship to inform its analytical framework, demonstrating the elasticity of security and illustrating the challenge faced in identifying its boundaries. In Chapter 2, Nasu locates security in customary international law and treaties, demonstrating the centrality of national security to international law. Against this backdrop, Chapters 3 to 5 show how international courts have dealt with security, including as a jurisdictional bar, in treaty interpretation, and in assessing when measures derogating from international obligations are justified. In Chapter 6, he considers how international institutions have used security to expand their remit, particularly in their response to crises, asking if this can be consistent with the international rule of law.

Nasu provides ample evidence that there needs to be more consistency in how international judges deal with security issues. He rightly explains that this reflects the difficult position those adjudicating security claims face: they must weigh the importance of international actors having discretion in emergencies against the public interest in ensuring legal accountability. To this end, his focus on case law is productive, showing the complexity of identifying when the invocation of ‘security’ in specific situations is appropriate and how, without care, courts can use what purport to be the same legal tests to justify radically different outcomes.

Nasu ends his book by arguing that judicial consistency is crucial for international law to justify solutions to normative problems through the objective application of legal rules. He identifies three principles that have the potential to promote consistency by serving as general standards for assessing security claims: requiring security claims to be made in good faith, ensuring that responses to security concerns comply with the principle of legality, and requiring that the means of protecting security are proportionate to the threat. International lawyers would do well to adopt these principles and, in doing so, find a better balance between ensuring that international actors have the necessary discretion to respond to security threats and maintaining legal oversight and the international rule of law.