The transcending of borders marked a large part of the political experience of the 20th century. This aspiration took shape in a century that strengthened and multiplied national barriers and that witnessed two world wars. In response to growing nationalism, many individuals, political movements and communities adopted an enlarged identity bound up with cosmopolitanism, internationalism, pan-Europeanism or, more recently, globalism. These categories, often part of a broader political frame of reference, established a connection between estranged nations but still referred to national divisions. Transnational history has shown how these tendencies were experienced not as a withdrawal of the border framework but as a link between nations-states. In this context, a particular and powerful way of abolishing national constraints has remained unnoticed by historians: the community of Esperanto speakers. They emerged at the end of the 19th century as an alternative to internationalizing models. With this aim, they developed a linguistic and cultural space that would not be marked by one specific culture to the detriment of the others. Nevertheless, the international language and the social movement created around it had to face the dominant ideological trends of the time. Through three case studies from the history of the Esperanto movement we propose to deepen the debate on the links between ideology and transnationalism. The first investigates the epistolary relationship between two communists, one Bulgarian and the other Japanese, discussing the trajectory of their political life and most notably their rejection of Stalinism. The second focuses on the link between Esperanto and anticolonial thinking in East Asia during the 1930s and early 1940s. Based on the study of literature and in particular of post-war Esperanto poetry, the third contribution examines the reasons that led a number of authors to choose the international language Esperanto to formulate their own account of WWII and to perpetuate the memory of the collective tragedy.