As Republican presidential front-runner Donald Trump has vaulted to the top of the polls, he has promoted controversial ideas about immigration – including the mass deportation of people in the United States illegally.

Some may think his statements are political grandstanding. But in relatively recent history, the United States government sanctioned policies that deported Mexicans and Mexican-American citizens.

With the stock market crash in 1929, unemployment skyrocketed, reaching a peak rate of 25 percent in 1933. The federal government searched for a solution. Mexicans and Mexican-Americans provided a useful scapegoat.

Under the positive-sounding name of “repatriation,” millions of people of Mexican descent were deported in a nominal effort to secure jobs for “real Americans.”

It didn’t matter if these people had American citizenship, says Francisco Balderrama, a historian at California State University, Los Angeles, who co-wrote a book on Mexican repatriation.

"There was a perception in the United States that Mexicans are Mexicans," Professor Balderrama told NPR. "Whether they were American citizens, or whether they were Mexican nationals, in the American mind, that is, in the mind of government officials, in the mind of industry leaders, they're all Mexicans. So ship them home."

The deportation wasn’t the result of one policy, but a combination of federal, state, and local policies that made it harder for people of Mexican descent to get or keep a job. Those policies included restrictions on employment in the public sector and limits on immigrant labor.

The idea was to create a wave of “self-deportation,” to borrow a phrase from Mitt Romney during the 2012 presidential campaign.

But sometimes officials turned to more forceful methods.

Olvera Street in downtown Los Angeles – as it stands today – is a hub for Mexican-American culture, with vendors selling colorful hand-woven blankets and steaming tamales. In February 1931, though, it was the site of a raid in which federal immigration officials, armed with guns and batons, swept through the area. The incident kicked off a spate of raids and removals nationwide.

In all during the 1930s, up to 2 million Mexicans and Mexican-Americans were sent to Mexico. According to scholars, as many as 60 percent were US citizens.

Kevin R. Johnson, dean of the law school at the University of California, Davis, laid out the lessons learned from Mexican repatriation in an article in the Pace Law Review.

“It is clear today that the conduct of federal, state, and local officials in the campaign violated the legal rights of the persons repatriated, as well as persons of Mexican ancestry stopped, interrogated, and detained but not removed from the country,” he wrote.

The campaign broke up families and sent children to Mexico who had lived in the US their entire lives, many of whom could not speak Spanish.

In 2006, lawmakers in California passed the Apology Act for the 1930s Mexican Repatriation Program, which expressed contrition to the deportees “for the fundamental violations of their basic civil liberties and constitutional rights during the period of illegal deportation and coerced emigration."

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Esteban Torres, a Los Angeles community leader and former US congressman, is the son of one of “los repatriados,” as the deportees became known. When he was a small child in Arizona, his Mexican immigrant father was rounded up from his workplace in a mine and shipped to Mexico.

"I was 3 years old. My brother was 2 years old. And we never saw my father again," Mr. Torres told NPR. "It was a sorrowful step that this country took."