The classical Aristotelian tension between "good person" and "good citizen" is no less salient in the present day than in Aristotle's time: in any imperfect political order, commitment to the laws and the regime's aims will conflict with commitment to a more comprehensive ethical order. The tension persists in liberal democracy in part because the regime's abstention from claims about the human good creates a problem of justification; liberalism has difficulty motivating the adherence of citizens on grounds apart from conventionalism. This dissertation seeks resources for justifying civic loyalties and resolving ethical-political conflicts, and it finds a valuable alternative to contemporary approaches in the ethico-political thought of Thomas Aquinas.Chapter 1 explores the contemporary landscape and develops the problem of justification. Chapters 2 and 3 examine Aquinas' comprehensive vision of the human good and the aims of political life, in order to highlight the tension between his perfectionist ethics and non-ideal regimes. Chapter 4 argues that Aquinas nonetheless offers unique resources for resolving ethical-political conflicts because his justification for politics is anchored in the same anthropological principles as his ethics: justice itself, a virtue constitutive of the human good, requires commitment to one's political community. The task of integrating civic loyalties into a unified life falls to citizens and their exercise of political prudence on a case-by-case basis. By shifting the referent for citizenship from the ruling regime to the political community as a whole, Aquinas depicts political prudence as a virtue that remains regime-relative even as it presses the regime in the direction of more complete justice. Chapter 5 argues that the dependence of political prudence on the moral virtues requires a civic education that points beyond the regime even while supporting the partial goods it embodies.Aquinas' virtue of political prudence offers a valuable model for defending commitment to existing regimes while correcting for their inevitable deficiencies. Yet the fact that his justification for good citizenship flows from the same philosophical anthropology as his broader ethical thought also presses political theorists to reflect on the implicit anthropological underpinnings of their own theories of civic virtue and education.