This chapter recovers the political thought of Constantin Frantz (1817–1891), a neglected conservative Prussian theorist of federalism. A contemporary of Tocqueville and Proudhon, Frantz argued that Germany and Europe’s stability depended not on building centralized nation-states but on nurturing a polycentric, subsidiarity-based federalism. Reinterpreting the Holy Roman Empire as a durable ‘federal union’ rather than a failed proto-state, Frantz condemned Bismarck’s Reich as a Bonapartist, homogenizing distortion of Germany’s true federative vocation. Against national solutions, he envisioned a European ‘federacy’: a renewed German Confederation embedded in northern and southern leagues, which would accommodate Polish independence and Italian unity within a supranational constitutional order. Strategically, this design aimed to check Russian expansion and preserve Europe’s autonomy against rising American power. The chapter also traces Frantz’s influence on strands of Central European conservatism and the interwar/postwar Abendland milieu, before explaining his eclipse in the Cold War. It concludes that his legacy remains relevant: Frantz highlights a distinctively Germanic-European tradition of ‘subsidiary federalism’, distinct from the American tradition of ‘centralised federalism’. Revisiting his vision today helps illuminate the EU’s polycentric character—an order where unity should be achieved through negotiated diversity rather than centralized command.