Making nature protection a matter of international concern was not an easy task in the 1920s and 1930s. It confronted the staff of the Office with a broad set of questions: Which nature was internationally relevant? Who were the experts that would provide knowledge about its actual state? Which local and national networks could be mobilized for its protection? And how to relate policies on the national, the European and the global level in a time period of strong borders? My paper will discuss the intricacies that shaped the discussions on these issues – and that in many ways continue to influence debates on European and global nature conservation. To do so I will draw on both official meeting reports and personal correspondence, preserved in the Amsterdam City Archive. Currently, nature conservation might be a domain with a relatively clear object, respected experts and authoritative policymakers. All these things were, however, still matters of negotiation in the Interwar period. By focusing on these negotiations, my paper hopes to open the black box of present-day nature conservation.