The Rhine has long occupied a central place in the geopolitics and cultural imaginaries of Europe, alternately claimed as France’s frontière naturelle and Germany’s Father Rhine. Flowing from the Swiss Alps to the North Sea, the river functioned not only as a vital commercial artery but also as a symbolic frontier of sovereignty. From Louis XIV’s annexation of Strasbourg in 1681 to successive French occupations in the modern era, it embodied the ambitions of French territorial expansion. The doctrine of natural borders, articulated by Richelieu and revived during the Revolution, treated rivers and mountains as providential limits, while nineteenth-century historians such as Augustin Thierry and Camille Jullian sanctified the Rhine as ancestral Gallic territory, thereby legitimizing expansion in the language of restoration. In contrast, German nationalism coalesced around the Rhine Crisis of 1840, when French claims to the left bank provoked mass patriotic mobilization. Cultural productions—including Die Wacht am Rhein, Lorenz Clasen’s Germania auf der Wacht am Rhein, and the completion of Cologne Cathedral—recast the river as Germany’s defensive bulwark and cultural heartland. Lucien Febvre’s Le Rhin (1935) deconstructed the nationalist mythologies attached to the river, recasting it not as a “river of blood” or an immutable frontier, but as a border imaginary—conceived as a connective axis of trade, culture, and exchange that unsettled exclusive national claims and re-situated the Rhine within a shared European space.