Contemporary theories of cosmopolitanism associate it with a fundamental commitment to dialogue, tolerance and ever-widening spheres of community and communication. It is opposed to other more conflictual values and ideologies, such as particularism and nationalism, and nourishes teleological histories of ‘civilization’ and pacification. This paper questions such easy assumptions about cosmopolitanism by tracing its relationship with conceptions of civil conflict, focusing particularly on the age of Enlightenment as an era of conspicuous cosmopolitanism and proliferating civil wars. Cosmopolitanism and civil war were linked by the idea of the civitas, the organized human community which secures peace but which, as Roman authors from Cicero to Augustine had reminded later generations, was also the arena of recurrent and destructive conflict among fellow citizens or cives. This Roman tradition implied that to live in a civitas was to be prone to civil war; indeed, only the civilized could suffer civil war. Cosmopolitanism may have been envisaged as a salve, even a solution, for such conflict within civitates but its universalism had paradoxically unintended consequences. By expanding the boundaries of the civitas, cosmopolitanism extended the arena of civil conflict to encompass all humanity: as Marius Pontmercy asks in Victor Hugo’s Les Misérables (1862), ‘Civil war ... What did the words mean? Was there any such thing as “foreign” war? Was not all warfare between men warfare between brothers?’. And by implying that all humans were citizens of a single community, it also made thinkable ideas of ‘global civil war’ elaborated by Carl Schmitt and his followers and revived more recently by analysts of transnational terrorism.