Ethnic, nationalist, and religious conflicts and debates about international intervention have been central global preoccupations of the past hundred years. Such debates, this volume argues, were first framed in their modern form during the interwar period, when a "Modernist break" (akin to that in literature, philosophy, and the arts) transformed the way such conflicts were viewed. Internationalists began to cast identity-based claims -- whether those of anti-colonialists or European separatists -- not only as mortal dangers to international order but as indispensable to its revitalization....