By constructing the first transnational and interlingual conceptual history of ethnicity, Ethnos of the Earth reveals the pivotal role this concept played in the making of the international order. Rather than being a primordial or natural phenomenon, ethnicity is a contingent product of the twentieth-century transition from a world of empires to a world of nation-states. As nineteenth-century concepts such as 'race' and 'civilisation' were repurposed for twentieth-century ends, ethnicity emerged as a 'filler' category that was plugged into the gaps created in our conceptual organisation of the world. Through this comprehensive conceptual reshuffling, the governance of human cultural diversity was recast as an essentially domestic matter, while global racial and civilisational hierarchies were pushed out of sight. A massive amount of conceptual labour has gone into the 'flattening' of the global sociopolitical order, and the concept of ethnicity has been at the very heart of this endeavour.