This dissertation engages the historiography of American citizenship and identity, republican traditions in American life and thought, and explores the evolution of military leadership in American society during the American Civil War. The nature, experiences and evolution of citizen-soldiers and citizen-officers, both Union and Confederate, reveal that the sentimental, often romantic expectations and ideologies forged in the American Revolution and modified during the antebellum era were recast, adapted, and modified under the extreme pressures of four years of conflict. Civil War citizen-officers experienced extreme pressures to emulate the professional officers of the regular army and to accommodate the ideological expectations of the independent, civic-minded volunteers they led. These junior leaders arrived at creative, often ingenious solutions to overcome the unique leadership challenges posed by the tension between antebellum democratic values and the demands of military necessity. Though the nature and identity of the officers in both armies evolved over time, the ideological foundations that informed Civil War Americans’ conceptions of military service persisted throughout the conflict. The key to the persistence of the citizen-soldier ethos and citizen-officer image during and after the Civil War era lies in the considerable power of antebellum Americans’ shared but malleable republican tradition. By focusing on the experience of volunteer company-grade officers in the Civil War era, we discover how the ordeal of the Civil War forced Americans to reevaluate and reconcile the role of the individual in this arrangement, both elevating and de-emphasizing the centrality of the citizen-soldier to the evolving narrative of American identity, citizenship, and leadership.