In this book, Campbell McLachlan KC provides a detailed and contemporary view of how international legal rules work together. This is a salient study as the rules are becoming more numerous and complex. The book examines the rules themselves and considers the idea that international law operates as a single system, even if it does not always appear that way. McLachlan is a barrister and academic from New Zealand who has worked with the International Law Commission’s Study Group on Fragmentation, especially on Article 31(3)(c) of the Vienna Convention on the Law of Treaties (VCLT). This work serves as the basis for this book, which builds on his idea of systemic integration, encompassing what people have said and written on this subject over the last 20 years. His main focus is on what Article 31(3)(c) VCLT means, and that when interpreting treaties, there is a need to consider any relevant rules of international law that would apply as between the countries involved. McLachlan says that this rule shows a wider idea of systemic integration: when many rules could apply to a disagreement, they should be understood as working together, not separately or against each other. He believes this is not simply a technical rule for understanding treaties but rather that international law aims to be a single, united system.

The book first examines the rules behind systemic integration, places them within the ILC’s project on fragmentation, and traces how courts have received this, such as the ICJ’s Oil Platforms case.Footnote ¹ Second, it explores the different ways that rules can relate to each other by way of conflict, overlap, and integration and scrutinises how courts have used systemic thinking in real instances. Third, it asks a bigger question: can international law really be called a system?

McLachlan discusses Hersch Lauterpacht’s view of international law as a scientific system and replies to people who disagree, noting that it doesn’t have a clear order. Instead of seeing fragmentation as a bad thing, he states that integration is a practical skill that courts use to balance different ideas and retain some unity. This is one of the most useful parts of the book. It changes the discussion from concern about fragmentation to providing clear ways of considering it. The book is noteworthy because it looks closely at cases about human rights, investment, trade, and sea laws. McLachlan avoids a reduction to generalities. Instead, he provides different types of problems and ways to deal with them legally. But sometimes, saying that systemic unity is desirable might seem too hopeful, especially because countries currently often compete for power and disagree in international law. Some readers may want further discussion on more radical ideas, and they may wonder if integration is always a good thing.

Still, this is a reliable and innovative study that greatly enhances what has been written about understanding treaties and maintaining consistent legal ideas. It complements earlier works on fragmentation by focusing on solutions instead of focusing on problems. McLachlan gives both a way of understanding the ideas and a guide to dealing with the complexity of modern international law.