This article engages with the history of international investment law in the first half of the twentieth century. It traces how international lawyers inscribed their vision of an international legal order protecting private property of Western companies against attempts at nationalization in the wake of socialist revolutions and the decolonization of large parts of the world. The article focuses on the role of ‘general principles of law as recognized by civilized nations’ as building blocks for an international legal order today called international investment law. Rather than describing a direct line between contemporary standards of protection and the invocation of general principles, the article develops conditions of possibility for the emergent field of international investment law. These conditions are located both in arbitral practice, as well as in international legal scholarship of the early twentieth century. Based on the analysis of such arbitrations over disputes resulting from concession agreements and scholarly writings in the interwar period, the contribution draws out the modes of authorization upon which the legal claims advanced by international lawyers rested. At the heart of the vision were ideas of ‘modernity’, ‘civilization’, ‘equity’, and ‘justice’ that enabled a hierarchization of difference, locating Western claims to legality above rivalling claims of socialist and ‘newly independent’ states. These ideas ultimately constituted the paradox of a ‘modern law of nature’ that claimed timeless universality while authorizing the ordering of foreign property in line with Western conceptions of modernity.