This dissertation provides a history of non-citizen detention and exclusion in the second half of the 20th century, from the war on Communism to the war on crime. It proposes that detention and exclusion are intimately related forms of border policing—two strategies that governments use to force non-citizens out of the country. Sometimes, this relationship was indirect: when one strategy became difficult, the other became more attractive. This dynamic underscores a difference between immigrant detention and its criminal counterpart. This study highlights executive decision-making by focusing on a short time period— between 1950 and 1996—in which the law surrounding immigration detention did not change, but the executive’s detention policy often did. It examines the factors that drove the Eisenhower administration to limit its use of detention in 1954 and those that caused the Reagan administration to champion detention in 1981. A crucial change during these years was the passage of refugee laws that gave everyone—even those seeking refuge from American allies—the right to apply for asylum. Indeed, as this dissertation shows, a growing hostility towards asylum-seekers fueled both detention and exclusion, culminating in the passage of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996. This dissertation also elucidates the complexities of resistance, especially legal resistance, to immigrant confinement. It highlights the stories of lawyers who represented non-citizens as well as resistance that came from opponents of immigration and detainees themselves. But it also suggests that legal challenges to exclusion or deportation sometimes led to more detention and that challenges to detention sometimes spurred the creation of new methods of exclusion.