By the start of the 20th century, the scientific study of race had extended its influence into both domestic and international policy. For example, global refugee flows changed as eugenics specialists formulated the US Quota Act. Scientific racism was simply a younger cousin of a century-long project to categorize, classify, and label humans—creating a taxonomy of the species as scientists had for so many others. Those “scientific” classifications were on full display as the League of Nations worked to arbitrate borders in post-war Ottoman territories. Territory would be allocated based on identity, consistent with European insistence on the centrality of the nation to modern sovereignty. Nation-states were to be filled with nations, and those nations had to be defined according to modern, scientific principles. Human categories would then determine territorial allocations. This paper will use the League of Nations archives and secondary literature to analyze three episodes in the interwar period in which the League regulated territorial assignments based on (often ambiguous) human classifications. The Greek-Turkish population exchange assumed the existence of two mutually exclusive segments of humanity, while its implementation illustrated the complications inherent in deciding the criteria for division. Assigning Mosul province began as an identity-related process in which territory would go to Turkey or Iraq depending on the affiliation of the majority of its residents. Despite their efforts to scientifically distinguish the two groups, it soon became clear that these categories had little meaning on the ground. The dispute between Turkey and the French government over the district of Alexandretta was similarly to have been decided based on the identities of the majority of its residents, a project that revealed the fluid and multiple identities of its people. European efforts to assign contested territories in former Ottoman lands relied heavily on theoretical notions of the scientific divisions of humanity. Those same theories, of course, were the ones that had provided the underpinnings of European privileges to control those regions in the first place. While the ideologies of scientific taxonomies and racism were proved inadequate and problematic in the Middle East--and even though the project of imposing borders had been predicated on those same theories--the often disastrous new territorial assignments nonetheless succeeded in providing greater European access to the region.