Reflecting on this change, in 1958 the Supreme Court, in Leng May Ma v. Barber, held that “physical detention of aliens is now the exception, not the rule,” pointing out that “certainly this policy reflects the humane qualities of an enlightened civilization.”

How did we move so far away from the notion that detention — and not just family separation and tear-gassing — is inhumane? How did holding more than 40,000 migrants behind bars come to be seen as normal?

The answer lies with the migration of Cubans and Haitians in the 1980s. The change was prompted by the Mariel Boatlift, when approximately 125,000 Cubans fled for the United States in a highly politicized migration that garnered national and international media attention and inspired fear among many Americans. The unfolding of these events can be seen in the stories of individuals like Alberto Herrera, who shared his experiences with Mark Hamm, a professor of criminology at Indiana State. During the 1980s, Americans came to see Mr. Herrera and hundreds of other Cuban exiles as such a dangerous threat that they once again turned to detention as an acceptable way to manage immigration.

Born in Havana in 1952, Mr. Herrera was the eldest of seven children. Like many an elder sibling, he felt responsible for his family’s well-being. But life in Cuba was hard and he was regularly unemployed. His family often went hungry. At the age of 18, he decided to do something about it: He walked into a government grocery store in Havana, hid two pounds of goat cheese under his jacket and fled. He did not get far. He was arrested and imprisoned for seven years. When he was released, he was even less prepared to find a job than he had been before spending time in prison. On Sept. 21, 1979, less than five months after his release, Mr. Herrera was sentenced to 13 years for stealing two pairs of pants.

But on June 24, 1980, Mr. Herrera’s luck seemed to change. That day, a guard came to his cell and told him that he was going to be placed on a boat headed to the United States. Mr. Herrera didn’t know it, but two months earlier, Fidel Castro’s government had announced that anyone who desired to leave Cuba could depart from the port of Mariel. In response, thousands of Cuban émigrés who were living in Florida set out in shrimpers, yachts and sailboats to pick up their relatives. When the émigrés arrived at the port of Mariel, however, the Cuban government informed them that if they wanted to take their relatives to the United States, they also had to transport anyone else the government wanted them to take. On some of those boats, government officials forcibly placed mentally ill individuals and prison inmates, including Mr. Herrera.