Abstract
<jats:p>The article aims to provide a analysis of the evolution of the Intermarium and Three Seas Initiative concepts, tracing their historical origins, ideological continuity, and influence on shaping the strategic interaction between Ukraine and Poland from 1918 to 2025. Particular attention is devoted to the transformation of Józef Piłsudski’s interwar federalist project, its reinterpretation within the Polish émigré intellectual milieu (especially in the works of Jerzy Giedroyc), and its contemporary institutionalisation within the Three Seas Initiative and the Lublin Triangle. The study seeks to reveal how these conceptual frameworks shaped political decisions, security priorities, and regional strategies of Kyiv and Warsaw. For the first time, the article demonstrates Intermarium and Trimarium as successive stages within a single intellectual tradition that underpins the political imagination of Central and Eastern Europe. It argues that Prometheism, Giedroyc’s doctrine and modern regional formats represent different phases of one conceptual continuum aimed at strengthening regional subjectivity and ensuring Ukraine’s independence. The study reveals that, in the twenty-first century, Ukraine is no longer a peripheral participant but becomes one of the central actors of the Baltic–Black Sea region, for whom regional platforms constitute instruments of security, infrastructural integration and geopolitical agency. The analysis confirms that, despite diverse historical contexts, the concepts of Intermarium and the Three Seas Initiative share a common objective: creating a stable and cooperative space within Central and Eastern Europe. They have shaped the dynamics of the Ukrainian-Polish strategic partnership, contributed to overcoming historical disputes and enabled the formation of a new model of regional security. The institutional formats of the Three Seas Initiative and the Lublin Triangle demonstrate a shift from interwar «utopian federalism» to pragmatic infrastructural and security regionalism. For Ukraine, these processes signify a transition from a buffer-state role to an active regional power capable of influencing the architecture of the Baltic–Black Sea–Adriatic area. Keywords: Intermarium, Three Seas Initiative, Trimarium, Prometheism, Lublin Triangle, Central and Eastern Europe, Ukrainian-Polish relations, Baltic-Black Sea region.</jats:p>