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 deposited there by American immigration officials. They had been picked up in the United States where they had homes and jobs and left in 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.

With the help of the Mexican government, which sought the return of Mexican nationals to alleviate a labor shortage, Border Patrol agents and local officials used military techniques and engaged in a coordinated, tactical operation to remove the immigrants. Along the way, they used widespread racial stereotypes to justify their sometimes brutal treatment of immigrants. Inside the United States, anti-Mexican sentiment was pervasive, and harsh portrayals of Mexican immigrants as dirty, disease-bearing and irresponsible were the norm.

During Operation Wetback, tens of thousands of immigrants were shoved into buses, boats and planes and sent to often-unfamiliar parts of Mexico, where they struggled to rebuild their lives. In Chicago, three planes a week were filled with immigrants and flown to Mexico. In Texas, 25 percent of all of the immigrants deported were crammed onto boats later compared to slave ships, while others died of sunstroke, disease and other causes while in custody.

It’s not clear how many American citizens were swept up in Operation Wetback, but the United States later claimed that 1.3 million people total were deported. However, some historians dispute that claim. Though hundreds of thousands of people were ensnared, says historian Kelly Lytle Hernandez, the number of deportees was drastically lower than the United States reported—likely closer to 300,000. Due to immigrants who were caught, deported, and captured again after re-emigrating, it’s impossible to estimate the total number of people deported under the program.

Mass deportations of Mexican immigrants from the U.S. date to the Great Depression, when the federal government began a wave of deportations rather than include Mexican-born workers in New Deal welfare programs. According to historian Francisco Balderrama, the U.S. deported over 1 million Mexican nationals, 60 percent of whom were U.S. citizens of Mexican descent, during the 1930s. Balderrama toldFresh Air’s Terry Gross that the program was referred to as “repatriation” to give it the sense of being voluntary. In reality, though, it was anything but.

Despite a widespread belief among native-born Americans that Mexicans came to the United States to steal jobs from American workers, many were invited to the country to work in its fields. In 1942, the U.S. Mexican Farm Labor Program, also known as Operation Bracero after the Spanish term for “manual laborer,” began. The program funneled Mexicans into the United States on a legal, temporary basis in exchange for guaranteed wages and humane treatment—an attempt by the Mexican government to stave off the discrimination faced by earlier immigrants.

However, not all employers wanted to follow the guidelines or pay the thirty-cent-an-hour guaranteed wage (about $4.51 in modern dollars). Nor did the Mexican government want Mexicans to work in Texas, which continued its discrimination against Mexican people, and the state was excluded from the program between 1942 and 1947. That’s where “wetbacks” came in. The racial epithet was used to describe Mexicans who illegally entered Texas by crossing the Rio Grande River. The government turned a blind eye to Texans’ employment of these undocumented immigrants, even after hiring undocumented workers was declared illegal.

An estimated 4.6 million Mexicans entered the country legally through the Bracero Program between 1942 and 1964, and states like California soon became dependent on bracero workers. At the same time, hundreds of thousands of Mexican workers crossed the border without permission and found jobs on the farms of employers willing to flout the law.

In 1953, the government decided it had had enough. By refusing to participate in the Bracero Program,  South Texas farmers essentially received their labor for less money than farmers who complied. And Border Patrol head Harlon B. Carter—a convicted murderer who killed a Latino as a teenager in 1931 and who later headed the National Rifle Association (NRA)—was frustrated by the sheer numbers of Mexican immigrants, both legal and undocumented, in the United States. He convinced President Eisenhower to ramp up immigration enforcement efforts.

In 1953, Carter tried to get the National Guard involved in a forerunner of Operation Wetback,, but since the U.S. military is not supposed to be used to enforce domestic laws, he couldn’t gain authorization to do so. Instead, in 1954, the government introduced Operation Wetback, which used Border Patrol resources instead.

Operation Wetback may not have had troops, but it used military tactics and propaganda to achieve its goals. It was headed in part by General Joseph Swing, head of the Immigration and Naturalization Service, and was planned like a war strike.

In an interview with the Los Angeles Times, Carter promised sweeps of factories, farms and other workplaces, ending with the detention of undocumented workers in holding facilities to await deportation. It would be “the biggest drive against illegal aliens in history,” Carter told the paper. News of the raids terrified Latinos in the United States, many of whom remembered the wave of forced deportations in the 1930s.

Historians have documented the behind-the-scenes wheeling and dealing that the United States participated in during Operation Wetback—deals that were not publicized at the time. Immigration officials threatened South Texas employers, some of whom had resorted to hiring armed guards to fend off Border Patrol officers, with stepped-up raids and offered them watered-down versions of the Bracero Program that let them get papers for their workers without committing to all of the program’s strenuous requirements. As a result, the number of immigrants in the Bracero Program grew as undocumented workers were deported.

Operation Wetback “was lawless; it was arbitrary; it was based on a lot of xenophobia," Hernandez told CNN in 2016. "And it resulted in sizable large-scale violations of people’s rights, including the forced deportation of U.S. citizens."