The United States Disciplinary Barracks at Fort Leavenworth, Kansas is America’s only maximum-security military prison. Despite frequent references to it in American popular culture, little is known about Leavenworth’s evolution as an institution. The existing sources on the subject tend to offer a factual account of which buildings were built when, and when certain vocational programs began operation, but they offer little analysis of the wider factors that influenced the development of the institution. This thesis argues that the evolution of Leavenworth into a permanent military institution has been driven by certain key events that have forced the intersection of the civilian and military spheres. By supplementing the analysis of military sources with that of unexamined civilian sources, such as memoirs from within the prison and newspapers, a broader understanding of the civilian pressures which helped drive Leavenworth’s evolution is achieved. This thesis also relies on secondary literature to explain and analyse the wider context in which the prison existed, particularly the study of civil-military relations and its relevance to Leavenworth. Fort Leavenworth did not develop within a vacuum of military thought, but rather was influenced by key events that forced an intersection between the civilian and military spheres. The result of these changes has been the evolution of a military institution that has remained separate from the civilian penal system, while at the same time adopting some of the more functional methods of that system.