The Congress of European Nationalities (hereinafter, the Congress) was the only organization in the interwar period that aimed to create an international forum encompassing all organized national minorities in Europe. In the fourteen years of its existence, the Congress continued to see itself as a unified movement despite several national minorities turning their back on it due to internal conflicts. In this study, I first describe the organizational structure of the Congress and analyze its official and public activities. I then discuss the causes of tensions and the complexities of cooperation among the national minorities. I also examine the efforts of Germany and Hungary – the kin-states of the two most active national minority groups in the Congress – to use the institution to serve the interests of the Hungarian-speaking and German-speaking minorities.