Regimes experiencing democratic backsliding, now embedded within supranational entities like the European Union, no longer represent mere trends but constitute the current geopolitical context. These regimes challenge the legitimacy of the interstate system while simultaneously confronting pressures from the supranational level. This interaction creates a complex, multi-layered, and multi-faceted system of arguments and challenges, which this paper aims to dissectand clarify. Utilizing the case study method and employing the Bellamy-Castiglione model as its theoretical foundation, this study seeks to probe the depths of legitimacy as a concept. This study endeavors to investigate the intricate interactions and challenges faced by states within a democratized supranational framework, offering insights into the evolving landscape of international governance. The presented analysis approaches the fields of political philosophy and legal theory, but only to indicate academic boundaries. The paper aims to enrich the discussion on the core and future of interstate relations amid inherent dynamics.