When Americans hear the name John Phillip Sousa, they likely equate him with brass band music and the Fourth of July holiday. Neither theatre scholars nor audiences generally link Sousa with musical theatre; however Sousa saw nine of his comic operas and one musical comedy produced between 1879 and 1913. This dissertation is primarily a work of musical theatre history; however, it argues that Sousa's comic operas were constructed to play a role in how American identity was manufactured and disseminated at the turn of the twentieth-century by reflecting new definitions of the "American" from the stage and circulating these new definitions nationally and internationally. Sousa constructed himself into an American cultural icon, a "Salesman of Americanism," during an era renegotiating national identity. His works, therefore, carry the weight of his iconic stature, casting their messages as an American' point of view. In addition to a comprehensive discussion of each operetta's form and production history, I argue that Sousa's comic operas can be cast as cultural ambassadors for social and political ideas; as musical theatre works attempting to re-define American identity in the eyes of audiences on both sides of the Atlantic. While situating Sousa's comic operas within the framework of musical theatre history, I argue that these cultural ambassadors were powerful agents advocating political and social change, intervening in the major social debates of the period, specifically in dialogues on race, foreign policy, copyright, labor, and suffrage. Because of his status as a great American,' and as a musical theatre composer rivaling Victor Herbert and George M. Cohan, the use of Sousa's operettas to sell ideology taught those watching that propaganda could be effectively integrated into musical theatre offerings. The musical theatre production - particularly on tour - served as one of the first forms of mass popular entertainment that could be used for political and social advantage. Sousa's comic operas, therefore, were a small part of the redefinition of the American musical, pushing its form toward integration, and shifting it from diversion and spectacle to ideological tool.