The 1920 Amsterdam Mahler Festival (the Mahler-Feest) was cast simultaneously as a celebration of Gustav Mahler’s life and works around a decade after his death, a jubilee honoring Willem Mengelberg on his twenty-fifth anniversary as director of the Concertgebouw Orchestra, and a grandiose return to public concert life following the First World War. In this dissertation, I argue that the festival’s organizing committee had yet another lofty goal: to turn this musical event into an unofficial diplomatic gathering, bringing artistic representatives together from across the Western world under the shared belief that the festival—and specifically the music of Gustav Mahler—would pave the way toward a more unified Europe after the turbulent years of the 1910s. Throughout this project, I analyze various elements of the Mahler-Feest through both musicological and political-historical frameworks, showing that every aspect of the festival was carefully designed to convey a spirit of internationalism and universality to those in attendance. Among these elements were the assembly of prominent guests from around the Western world, the performance of chamber music written by composers from various nations alongside the central program of Mahler’s works, the signing of a Manifesto of Foreign Guests promoting similarly politicized festivals in the future, and the establishment of a global Mahler Union that was to be headquartered in Amsterdam. I further demonstrate that the internationalistic aspects of the event also promoted an underlying nationalistic ideology, with the festival serving to support the diplomatic goals of the Dutch state, which sought to posit itself as a neutral site for dialogue and mediation among nations during the early decades of the twentieth century. Among the central figures in this dissertation is Rudolf Mengelberg—the Concertgebouw’s program annotator and a distant cousin of Willem—who, through his expansive program book written for the festival, casts Mahler as the composer whose music best matched the political framing of the event. To further analyze the Mahler-Feest, I compare this Mengelberg’s characterizations of Mahler with the viewpoints and beliefs that the composer expressed during his own lifetime, showing that Mengelberg took advantage of historical ambiguities to promote his politicized interpretations of Mahler without directly contradicting the documentary evidence available at the time. At the end of the dissertation, I assess the impact that the perspectives advanced at the festival have had (and continue to have) on the broader realm of Mahler scholarship across the past century, and I briefly examine the evolution of the Mahler-centric festival from 1920 through the present day. Methodologically, this study uses archival evidence to bring together lines of inquiry spanning the fields of musicology, political history, anthropology, and the emerging discipline of festival studies.