Why do some political systems adopt a federal form of territorial organization? This chapter argues that the emergence of federalism is closely linked to the foundational moments of a polity, and in particular, to the relative strength of the regional or national units involved in state formation. Federation, it contends, can only arise and endure as a delicate equilibrium between a “center” and a “periphery” of comparable power—where the central authority is not strong enough to impose its will unilaterally, and the regions are not strong enough to obstruct the federal integration process, including through the threat of secession. The chapter examines post–Second World War experiences of political decentralization in Europe, with a particular focus on Germany, Italy, and Spain. While all three cases are considered, special attention is devoted to Spain to illustrate how the “principle of weakness”—understood as a necessary condition for maintaining balance within a federal compact—helps explain the ongoing difficulties in establishing genuine federal structures, and especially a multinational federation, in the Spanish context. This framework sheds light on why federalism remains a challenging constitutional model to implement in Spain, despite sustained support from various political forces advocating for deeper decentralization and the recognition of sub-state nations.