Juan Olivarez was 24 when he first went to the United States in 1966. He entered as a tourist, but he really went to work. After returning to his small town in the central-western Mexican state of Jalisco the following year, he headed north again and found jobs picking beets, tomatoes and cherries in California. Olivarez and some friends later went to Oregon, where they repaired train tracks until, one night after dinner, an immigration officer knocked on their door and asked to see their papers. “We told him, almost in unison, ‘We don’t have any,’” Olivarez recounted. He spent the next week being shuffled between four different jails and detention centers in California and Texas before authorities finally deported him.

Olivarez’s expulsion is one of nearly 50 million deportations U.S. authorities have carried out during the last half century. Between the late 1970s and the late 2000s, annual expulsions exceeded one million, on average, and rarely dipped below 900,000 in any given fiscal year. Though the years before had seen the episodic enforcement drives, such as those of the Great Depression and the infamous “Operation Wetback” campaign of the 1950s, the magnitude and regularity of deportations after 1975 marked a break from the past and the dawn of a new era: the age of mass expulsion.

Like Olivarez, some 90% of the people pushed out of the country during the 20th century were Mexicans deported via a coercive, fast-track administrative process euphemistically referred to as “voluntary departure.” Similar to prosecutors in the criminal justice system relying on plea bargains, immigration authorities depended on voluntary departure, making it seem like the best of all the bad options facing people who had been apprehended. These deportations without due process also functioned as a cost-saving measure, since they minimized detention-related expenses and reduced the number of immigration hearings.

In the 1970s, immigration authorities’ near-exclusive reliance on voluntary departure, the United States’ dependence on cheap migrant labor and the border’s relative porousness combined to create a revolving door effect and a sense of perpetual crisis.