This article concentrates upon Hans Kelsen’s theory of legal monism, as presented in his Lecture delivered at the Académie de droit international, The Hague, in 1926. This Lecture propounds a legal theory which enables a critical engagement with the emerging legal order of the 1920s. The article draws upon recent scholarship regarding the international legal agreements adopted in 1924 (the London Accords) and 1925 (the Locarno Accords) to demonstrate their potential for a new form of European and international stability. This potential, however, remained constrained by the logic and limitations of the primacy of State sovereignty. Within this context, Kelsen’s Lecture sought to set the emerging international order within a unified legal framework of national and international positive law. This framework enabled Kelsen to identify two variants of legal monism, differentiated by the role of State sovereignty in the relationship between national and international law. For Kelsen, it is the choice between the broader ethico-political consequences of these two variants which will shape the further evolution of the global legal order.