In a speech at Madison Square Garden Sunday night, former President Donald Trump reiterated his pledge to “launch the largest deportation program in American history” on day one of a second term.

That raises two questions: If he wins the election, could he even do that? And if so, how would it work?

The answer to the first question is a little complicated. While presidents have broad powers over immigration, there are operational, legal, and political challenges associated with his plans that involve invoking an 18th-century legal authority that hasn’t been used since World War II. And though public support for the policy appears to be growing, it’s not clear Americans actually know what they’re asking for.

The answer to the second question is more straightforward: If Trump and his allies can overcome those obstacles, history provides a clear — and devastating — picture of how a federal mass deportation program might go.

The US has previously implemented mass deportation programs targeting Mexicans in the 1950s and during the Great Depression. But never has a deportation initiative targeted so many people, especially those who have lived in the US for years — or even decades — and have family here, than what Trump is proposing. For that reason, Trump’s plans may be even more disruptive than previous mass deportation programs, terrorizing families who have been here for years and tearing apart communities where undocumented immigrants have planted roots.

Here’s what this new iteration of mass deportations might look like, based on what we’ve seen before and what we know about Trump’s plans.

What have previous mass deportations been like?

The most prominent example of a wide-scale deportation program in US history is Operation Wetback, named after the racial slur used to describe immigrants who crossed the Rio Grande to reach the US southern border.

Spearheaded by President Dwight Eisenhower in the 1950s, the program used military-style tactics to round up undocumented workers (and, mistakenly, some US citizens) and cram them onto buses, boats, and planes headed for Mexico.

Many of those workers had come to the US under the Bracero Program, a government initiative that allowed them to legally work in the US agricultural sector on a temporary basis. But amid rising American anti-immigrant sentiment and the perception that the Bracero Program was fueling unauthorized immigration, the Eisenhower administration clamped down.

By the government’s estimate, as many as 1.3 million people were deported under Operation Wetback in the span of about a decade.

“They certainly succeeded in returning a lot of people. They certainly succeeded in disrupting labor markets,” said Doris Meissner, a senior fellow at the Migration Policy Institute and former head of the federal immigration agency previously called the Immigration and Naturalization Service, whose functions were eventually split among the three federal immigration agencies that exist today. “But in the process, lots of people actually were improperly deported who were US citizens or who did have some other right to be in the United States.”

Before Operation Wetback, there was a wave of mass deportations during the Great Depression. At a time when unemployment was high, peaking at over 25 percent in 1933, many Americans believed Mexican immigrants were taking their jobs.

Federal and state governments in the US reacted by initiating “repatriation” campaigns that involved raiding workplaces and public spaces and deporting anyone authorities perceived as Mexican, including US citizens.

Up to 2 million Mexicans and Mexican Americans were ultimately deported under the program in the 1930s and ’40s. This stoked fear in the Mexican American community, causing many others to leave the country on their own accord.

How would mass deportations work today?

In his speech at the Republican National Convention in July, Trump promised a mass deportation program even larger than Operation Wetback. But he didn’t elaborate on the specifics of his plans until a rally in Aurora, Colorado, earlier this month, when he announced that he intends to invoke the Alien Enemies Act, a 1798 law passed as part of the Alien and Sedition Acts. He is naming the plan “Operation Aurora,” after the city, which he has falsely portrayed as under siege from immigrant criminals.

The Alien Enemies Act allows the president to detain and deport noncitizens from countries at war with the US. It was last used during World War II to detain civilians of Japanese, German, and Italian descent. The US government later apologized for their internment and provided reparations to those of Japanese descent, but the law remained on the books — ready for Trump to pluck out of obscurity.

Trump has indicated that he intends to first target “known or suspected gang members, drug dealers, or cartel members.” That reportedly includes members of the Venezuelan gang Tren de Aragua.

The gang, which began in an infamous prison in the Venezuelan state of Aragua, has been linked to extortion, kidnappings, and drug trafficking in the US since nearly 8 million people have fled the country in recent years during the calamitous rule of President Nicolas Maduro. In July, the Biden administration sanctioned the gang, putting it on a list of transnational criminal organizations and announcing $12 million rewards for the arrests of three leaders.

Trump said in Aurora that, if he wins a second term, he would “send elite squads of ICE, Border Patrol, and federal law enforcement officers to hunt down, arrest and deport every last illegal alien gang member until there is not a single one left in this country.”

“And if they come back into our country, they will be told it is an automatic 10-year sentence in jail with no possibility of parole,” he said.

However, he and his running mate JD Vance have suggested that they would not just stop at gang members. When pressed for a number, Vance previously said they would set a goal of 1 million deportations. That would potentially encompass people who aren’t violent criminals and who have lived in the US for years if not decades. And that’s what makes Trump’s plans different and more devastating to the communities where these immigrants reside from previous deportation programs.

Experts have also raised concerns that, even more so than during Operation Wetback and the immigration raids of the Great Depression, US citizens (including American children of immigrants) could get caught in the fray. Tom Homan, Trump’s former director of US Immigration and Customs Enforcement and current immigration adviser, has said that families would be deported together, apparently including US citizens.

Deportations on the scale Trump is proposing (especially if they swept up US citizens) could result in knock-on effects — including to the economy — that Trump has not publicly discussed.

“The target population today is so much more varied and has been here for so much longer a period of time, and spans so much more geography as well as labor market areas and occupations,” Meissner said. “It would be much more disruptive and likely result in severe violations.”

Is Trump’s mass deportation plan actually feasible?

There are a lot of problems with Trump’s plan.

For one, it’s entirely impractical from an operational standpoint. The law enforcement capacity needed to both secure the border and carry out mass raids in the interior of the US simply does not exist. Getting US Immigration and Customs Enforcement, detention facilities, and immigration courts staffed to the levels Trump’s plan would need would require massive investment. That money would have to be approved by what may well be a divided Congress. If, as is projected, Democrats control the House of Representatives, any congressional funding would probably be off the table.

“Every possible institution involved in this is already hugely overburdened and would be pretty much crippled in trying to handle the workload,” Meissner said. “It’s just a recipe for institutional breakdown.”

Even if the capacity existed, any mass deportation program would likely rely on state and local law enforcement agencies, as well as the National Guard. But only state officials aligned with Trump, such as those in Texas and Florida, may be willing to activate those law enforcement capacities on his behalf.

“I expect that in a second Trump term, we would see migrant communities in Republican states take the heaviest hit,” said César Cuauhtémoc García Hernández, a professor at Ohio State University’s Moritz College of Law and author of the forthcoming book, Welcome the Wretched: In Defense of the “Criminal Alien.” “On the flip side, [there could be] a lot of foot-dragging, if not outright resistance, by states and cities and counties led by Democrats.”

Invoking the Alien Enemies Act might also be illegal. As Katherine Yon Ebright, liberty and national security counsel at the Brennan Center, notes in a recent report, the law has never faced a challenge under the Constitution’s guarantee of equal protection under the law. She writes that the law “covers noncitizens on the basis of their ancestry and is an overbroad and inefficient means of preventing espionage and sabotage in wartime,” suggesting that it could be overturned.

Trump would also have to make the case that he is justified in invoking a wartime power. The US is not currently at war, though Trump and his allies are trying to paint the picture that it is. In public remarks, Trump has said that the US is facing the “greatest invasion in history” at the southern border, that it must protect against an “enemy from within,” and that immigrants are “totally destroying our country.”

That said, it’s unclear whether any legal challenges are likely to be successful. The last time the Alien Enemies Act was challenged in court in 1948, a federal judge sided with the Truman administration. Ebright writes that the court was reluctant to overstep the president’s wartime powers in the period following World War II.

Trump may also find sympathizers on the federal bench: He stacked the courts with Republican judges during his first term and has a conservative majority on the US Supreme Court.

But if he goes through with his plans, Trump may also have to confront renewed political opposition. Voters have become more anti-immigration during the Biden administration, but if Trump pushes too far, he might find that trend reverses. Americans rallied behind immigrants and increasingly supported higher levels of immigration during his first term; Democrats may again organize themselves in opposition to his policies.

“I think the Trump administration, the second time around, could certainly breathe fear into millions of people around the United States,” García Hernández said. “But I think the more that they do that, the more that they’ll rile up the folks who under the Biden administration have really turned their attention to other matters.”