In this article I explore the cultural impact of the First World War by analysing the work of libraries and librarians in different settings, from German-occupied Belgium and prisoner-of-war camps to Germany's own public and private libraries. By examining the work of German, Russian, Polish, Ukrainian, and American librarians, the article makes a case for applying the notion of a ‘long’ First World War to cultural and intellectual history. Considering ephemeral and established libraries, along with new types of collections generated by the war itself, the article sheds light on the changes to library work as a result of mobilization, censorship, and the growth of mass readership. While these changes concerned librarians in all belligerent states, German librarians were particularly affected after the burning of the Leuven library during the city's occupation by German forces. This singular event damaged Germany's national reputation and thereby laid the groundwork for a significant politicization of library work all the way to the Second World War. In addition to tracing the importance of librarians for German foreign policy, the article reconstructs how this professional community, whose intellectual formation was ultimately supranational, responded to the First World War.