Ancient discourses on utopia range from nostalgic evocations of a peaceful Golden Age to the forward-looking projects of a well-ordered society free of internal violence. It is this second perspective that connects to the issue of how to maintain the fragile equilibrium between war and civil war. The idea of using the former as a remedy against the latter is present in the blueprint of the just city whose hypothetical generation is depicted in Plato’s Republic. Reading Plato with Thucydides enables us to see how both the diagnosis of civil strife’s causes and the (impossibly utopian) remedy connect to a certain underlying conception of man. Not unlike Platonic utopia, conceived as an artificial remedy to natural inner conflicts, some Hellenistic and Greco-Roman philosophies of peace share in the vision of an ever-expanding “circle” of the city. However, their understanding of human nature is different, and despite some complicity between cosmopolitanism and imperialism, they are more optimistic in considering the perfectible community not as an imposed artificial structure of justice but as a structure whose germ is naturally present in our individual constitution.