The Westphalian Peace Treaty is widely regarded as the foundation of the modern international order (the so-called ‘Westphalian system’), characterised by the co-existence of sovereign states which do not acknowledge any superior power. But as recent research has shown this interpretation to be quite misleading, the aim of this chapter is to investigate how and when this perception emerged in the first place. Although the Westphalian Peace originated in a universal congress, that congress led to a particular Central European security order, rather than a universal peace treaty. It was, in fact, from the mid-18th century onwards that a new, ‘universal’ interpretation of the Westphalian Peace gained traction in intellectual debates and in treaty-making practice in Europe through repeated affirmation, eventually even between states that had not been party to the Peace itself; this led to its portrayal as the foundation of a universal state system and the droit de gens. In other words, the powerful notion of the ‘Westphalian system’ can be traced back not only to the late 1940s (as hitherto presumed), but to the ideas of 18th-century diplomats, politicians and intellectuals.