The Baltic Sea has historically functioned as a borderland of fluid frontiers, where mobility, commerce, and cultural exchange blurred the lines of political sovereignty. The successive Germanic migrations of Late Antiquity from the southern Baltic littoral into Roman domains—supplying amber, iron, and mercenaries—illustrate how this maritime zone operated less as a fixed boundary than as a generative frontier. Viking seafaring expanded these networks, linking Scandinavia to Byzantium and the Islamic world and fostering hybrid polities such as Kievan Rus’. In the late medieval era, the Hanseatic League institutionalized this connectivity, embedding the Baltic within a wider transregional economy. Later, imperial rivals—Denmark, Sweden, and Russia—contested dominium maris Baltici, though none secured lasting supremacy. The Baltic thus persisted as a contested yet connective sea, producing diasporas, hybrid identities, and cross-border solidarities. As both corridor and frontier, it illustrates the central paradox of borderlands: simultaneously sites of division and of integration in Europe’s historical trajectory.