This dissertation analyzes the transformation of noncitizen detention policy in the United States over the twentieth century. For much of that time, official policy remained disconnected from the reality of experiences for those subjected to the detention regime. However, once detention policy changed into its current form, disparities between policy and reality virtually disappeared. This work argues that since its inception in the late nineteenth century to its present manifestations, noncitizen detention policy transformed from a form of exclusion to a method of state-sponsored violence. A new periodization based on detention policy refocuses immigration enforcement into three eras: exclusion, humane, and violent. When official policy became state violence, the regime synchronized with noncitizen experiences in detention marked by pain, suffering, isolation, hopelessness, and death. This violent policy followed the era of humane detentions. From 1954 to 1981, during a time of supposedly benevolent national policies premised on a narrative against de facto detentions, Arizona, and the broader Southwest, continued to detain noncitizens while collecting revenue for housing such federal prisoners. Over time increasing detentions contributed to overcrowding. Those incarcerated naturally reacted against such conditions, where federal, state, and local prisoners coalesced to demand their humanity. Yet, when taxpayers ignored these pleas, an eclectic group of sheriffs, state and local politicians, and prison officials negotiated with federal prisoners, commodifying them for federal revenue. Officials then used federal money to revamp existing facilities and build new ones. Receiving money for federal prisoners was so deeply embedded within the Southwest carceral landscape that it allowed for private prison companies to casually take over these relationships previously held by state actors. When official policy changed in 1981, general detentions were used as deterrence to break the will of asylum seekers. With this change, policy and reality melded. No longer needing the pretext of exclusionary rationales nor the fiction of humane policies, the unencumbered state consolidated its official detention policy with a rationale of deterrence. In other words, violence. Analyzing the devolution of noncitizen detention policy provides key insights to understanding its historical antecedents, how this violent detention regime came to be within the modern carceral state, and its implications for the mass incarceration crisis.