This magisterial new history elucidates a momentous transformation process that changed the world: the struggle to create, for the first time, a modern Atlantic order in the long twentieth century (1860–2020). Placing it in a broader historical and global context, Patrick O. Cohrs reinterprets the Paris Peace Conference of 1919 as the original attempt to supersede the Eurocentric 'world order' of the age of imperialism and found a more legitimate peace system – a system that could not yet be global but had to be essentially transatlantic. Yet he also sheds new light on why, despite remarkable learning-processes, it proved impossible to forge a durable Atlantic peace after a First World War that became the long twentieth century's cathartic catastrophe. In a broader perspective this ground-breaking study shows what a decisive impact this epochal struggle has had not only for modern conceptions of peace, collective security and an integrative, rule-based international order but also for formative ideas of self-determination, liberal-democratic government and the West.