The creation of institutions devoted to promoting and supporting modern music in the United States during the 1920s made it possible for American composers to develop an identity distinct from that of European modernists. These institutions were thus a critical part of the process of modernization that began in the United States during the early decades of the twentieth century. There is substantial scholarship on these musical institutions of modern music, such as the International Composers’ Guild and the League of Composers; but little to no work has been done on the progressive musical institutions of the 1910s, such as the Music League of the People’s Music Institute of New York, which was founded by Claire Reis. This thesis addresses the questions of how and why American musical modernism came to be as it was in the 1920s through an examination of the various stages of Reis’s career. The first chapter is an extensive study of primary source material gathered from the League of Composers/ISCM Records collection at the New York Public Library, which relates to Reis’s work with the PML in the 1910s. The second chapter uses the conclusions of the first chapter to shine new light on an old subject: the 1923 schism within the ICG that led Reis and others to form the League. The traditional view that the schism was the result of a conflict in idea of style is called into question, and the role that gender and power structure played in the break are explored.