Twenty years have passed since the author's delivery in 2000 of the general course of public international law at the Hague Academy of International Law, titled 'The Unity of the International Legal Order'. That course was designed to combat the all-too-common idea that international law was in the process of 'fragmentation'. It did so by developing a theory focused on the existence of and tension between two forms of unity in the international legal order: the formal unity (concerning the procedures by which primary norms are created and interpreted, and their non-compliance adjudicated) and the material unity (based on the content of certain norms of general international law, peremptory norms). Twenty years later, the time is ripe to revisit this theory to determine the extent to which it is still valid as a framework for the analysis of international law, particularly as an increasing number of 'populist' leaders very much seem to ignore, or voluntarily deny, the validity of some of the key substantial principles on which the international legal order was re-founded within and around the United Nations in 1945. When confronted with the factual reality of the present state of international relations as well as with the evolution of the law, one can conclude that the validity of the unity of the international legal order is unfailingly maintained, and that its role in upholding the international rule of law is more important now than ever.