July is scorching in Mexicali. The Mexican city just across the border from Calexico, California, averages 108 degrees Fahrenheit in the summer, but temperatures often swell into the 120s.

In 1955, thousands of disoriented people roamed the city’s streets as the sun bore down on them. They had just been dumped there by American immigration officials—snatched from their lives and jobs in the United States and thrown into a city where they didn’t know anyone.

These Mexican immigrants had been caught in the snare of Operation Wetback, the biggest mass deportation of undocumented workers in United States history. As many as 1.3 million people may have been swept up in the Eisenhower-era campaign with a racist name, which was designed to root out undocumented Mexicans from American society.

The short-lived operation used military-style tactics to remove Mexican immigrants—some of them American citizens—from the United States. Though millions of Mexicans had legally entered the country through joint immigration programs in the first half of the 20th century, Operation Wetback was designed to send them back to Mexico.