Focusing on nineteenth-century American military occupation, this dissertation critically engages the existing literature on Civil War soldiers. It departs from the traditional historiographical paradigm of “why they fought and endured”—based on motivation and the experience of active combat—and instead emphasizes how the soldiering experience was fragmented and fraught with disillusionment and confusion. The Civil War traditionally is interpreted as period-divide between the antebellum and post-bellum eras. Soldiers’ responses to the culture of military occupation, however, revealed striking continuity across time, space, and conflict in nineteenth-century America. By uniting three principal events—the Mexican-American War, Civil War, and post-bellum Reconstruction—the study interprets how nineteenth-century volunteer citizen-soldiers struggled to understand their roles as occupying forces. As occupation emerged as a fundamental staple of the American military tradition, its complexities challenged the cultural ideals that fueled the citizen-soldier model. The milieu of occupation thus contested American soldiers’ integrity, masculinity, and racial identity. The citizen-soldier tradition collided with an equally aggressive, and oftentimes incompatible, force: the garrison ethos. Volunteer soldiers confronted the principal tenets of military occupation—securing, holding, and guarding territory; enforcing government policies; regulating and defining the limits of civilian-combatants; policing cities and towns; and battling guerrillas—viewing them as trials against the citizen-soldier ideal, which they had intended to fulfill.