The prologue to this book zooms in on the inherent tensions and harmonies in the transatlantic relations that evolved in the first half of the twentieth century and laid the practical, ideational, and emotional foundations for the take-off of European integration as of 1950. In doing so, the prologue, in a more essayistic way, critically reflects upon the reconstruction of the history of the origins of European integration as presented in this book and the history of European integration in general, and the deeper meaning of both for our understanding of present-day Europe and the unique phenomenon of European integration. The prologue also introduces some key concepts and figures in the historical reconstruction that follows in the chapters, such as the the policies and politics of planning, the functionalism of David Mitrany, and the analysis of the vicissitudes of transatlantic relations by Isaiah Berlin.