In 2013, the United States detained approximately 400,000 people in immigration custody in a network of 250 local, federal, and private jails across the country as they awaited deportation or release, at a cost of over $1.7 billion. This dissertation situates the rise of the current U.S. immigration detention system in the early 1980s within the broader context of Ronald Reagan's Cold War foreign policies and growing public xenophobia after the Vietnam War. When President Reagan entered office, he sought new ways to curtail a perceived "mass immigration emergency" caused by an increasing flow of Cubans, Haitians, and Central Americans to the United States. As the American public continued to express "compassion fatigue" towards new migrant populations, the Reagan Administration established a new security state that included the building of immigrant detention centers throughout the United States, Puerto Rico, and Guantanamo Bay, Cuba; the interdiction of migrants on the high seas; heightened border security under the "War on Drugs"; and the first uses of private prison contracting. This work traces the narratives surrounding these new enforcement measures by using Reagan Administration files, media portrayals of migrant groups, and evidence of community and public support for and against the practice of immigration detention in order to demonstrate how an ongoing fear of future mass migrations continued to justify more permanent structures of immigration detention--trends that persist to the current day.