This article aims to show that concepts originating in the vocabulary of international relations were crucial to the rhetoric of nation-building in mid-nineteenth-century Europe. A close examination of the Romanian context elucidates in a more general way historical actors’ reflections and critiques of this conceptual vocabulary as well as the permeable nature of the (inter)national in the given historical context. The article explores two conceptual pairs:jus Gentiumversusjus publicum Europaeum, and sovereignty versus suzerainty. In the process, it shows how Romanian nation- and state-builders became scholars of international relations. This they did in an effort to demonstrate the historically grounded sovereignty of the Romanian Principalities, in a manner compatible with the prevailing norms of the law of nations. The emphasis on a contractual relationship with the Ottoman Empire allowed for the assertion of national agency, both in the past and in the present. Increasingly focused on the imperfect translatability of concepts forged by the Western historical experience, pamphleteers of all stripes ultimately came to jettison the supposedly feudal, anachronistic vocabulary of suzerainty, militating for the inclusion of the Principalities as full parties in European public law. Thus, the article elucidates some significant conceptual tensions in the development of mid-nineteenth-century nationalism, simultaneously contributing to a growing body of scholarship on the intellectual history of international relations.