Ernst Krenek's opera Karl V presents an ingenious interpretation of the life of the sixteenth-century Holy Roman Emperor, which argues not only dramatically but also musically for the contemporary political significance of the Emperor's life. This was no forced pairing of music and politics, for Krenek had found that the historical and theological problems raised by the Emperor's ‘justification’ at the end of his life bore a striking resemblance both to the aesthetic dilemmas he was then having to face as a composer and issues common to the wider struggle for national identity and political legitimacy in Austria after World War I. This essay introduces these themes in the work, and Krenek’s rationalisations of the way he presents them. It considers their implications for our understanding of the history of serialism in music more generally, and for an aspect of Krenek's compositional development that has perplexed later commentators: the apparent stylistic gulf between this opera and Jonny spielt auf.