Donald Trump's pathway to deporting millions of undocumented migrants may hinge on a 226-year-old law that was last used to detain non-citizens of Japanese, German and Italian descent during the Second World War.  

The 1798 Alien Enemies Act is a potential tool the U.S. president-elect has said he will use to try to make good on one of his key campaign pledges that otherwise could be stalled significantly by the legal machinations of the deportation process.

"If Trump were to try to use the normal procedures, it would [be to] round up a lot of people and put them into immigration court proceedings," said Stephen Yale-Loehr, a professor of immigration law at Cornell University.

"But it would be a long time before they could actually be deported."

According to the Center for Migration Studies, there were around 11.7 million undocumented migrants in the U.S. as of July 2023. 

Trump has said that on day one of his presidency, he will "launch the largest deportation program in American history." To that end, he recently announced that Tom Homan, who was acting head of Immigration and Customs Enforcement during Trump's first administration, would be his border czar.

Homan has previously said that he would be willing to "run the biggest deportation operation this country's ever seen."

Slow, expensive procedure

But because due process under the U.S. Constitution applies to everyone, not just citizens, those who have been accused of being undocumented migrants must go through immigration court proceedings, Yale-Loehr says.

During those proceedings, an immigration judge decides whether those individuals are deportable or have some relief from deportation, such as asylum, he says.

Currently, however, there is a backlog of 3.7 million cases in the immigration courts, according to the Transactional Records Access Clearinghouse at Syracuse University, which compiles statistics on immigration. Meanwhile, the U.S. Justice Department estimates there are only approximately 700 immigration judges in the country's 71 immigration courts.

"Many cases are being scheduled for four or five years from now," Yale-Loehr said.

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That means if Trump follows current deportation procedures, he will need money to hire more immigration agents, build more detention centres and hire more immigration judges, he says.

It could be an extremely costly endeavour. The American Immigration Council estimates that a one-time mass deportation operation, which would include expenses for arrest, detention, legal processing and removal — would cost more than $300 billion US.

A legal shortcut

That may be why Trump, in order to carry out mass deportations, may seek to circumvent the system by invoking the Alien Enemies Act, which was enacted when the U.S. and France were on the verge of war in the late 1700s.

Amid concern about potential French supporters living in the U.S. at that time, the law sought to to prevent foreign espionage and sabotage in wartime. It permits the president to target those individuals without a hearing, based only on their country of birth or citizenship, says Katherine Yon Ebright, counsel for the Brennan Center's Liberty and National Security Program.

The president may invoke the act in times of "declared war" or when a foreign government threatens or undertakes an "invasion" or "predatory incursion" against U.S. territory, Ebright recently wrote in a report for the Brennan Center, a nonpartisan law and justice institute.

Another requirement is that the invasion or incursion must be perpetrated by a foreign nation or government, Ebright noted.

The act has been used three times — the War of 1812, the First World War and lastly during the Second World War, when President Franklin D Roosevelt used it to deem Japanese, German and Italian non-citizens as "alien enemies" and arrest them.

Ebright says Trump and others have for years been trying to characterize unlawful migration and cartel activity at the southern border as an "invasion."

"They're saying, 'Well, because there's an invasion at the southern border, we can invoke the Alien Enemies Act against the perpetrators of that invasion. Then we can unlock that massive power to do summary detentions and deportations.'" Ebright told CBC News.

But she says the Brennan Center and other organizations are prepared to challenge Trump in court if he invokes the act, and would argue that it's being improperly invoked.

"There, in fact, is no invasion within the meaning of the law," she said.

"There is no foreign nation or government that is perpetrating this supposed invasion," she said, adding that gangs, cartels or undocumented migrants shouldn't be considered foreign nations or governments.

Yale-Loehr echoes that, currently, the U.S. has not made any declaration of war against immigrants and that Trump would have to, by analogy, say that trying to deport immigrants is akin to war.

Hiroshi Motomura, the faculty co-director of the Center for Immigration Law at UCLA, says the text of the Aliens Enemy Act doesn't seem to apply to this situation.

Motomura says the so-called invasion wouldn't be referring to people who are just showing up in caravans at the border, but to people who have been in the U.S. for a long time.

"If there was an invasion, and i don't think there was, it was 10 years ago. Or something like that," he said.

However, Ebright did say that courts could decide these are political questions that are outside the scope of what the courts can resolve, meaning Trump's argument could prevail.

But he would still face the same logistical challenges, Yale-Loehr said.

"You're still going to have to have some place to detain all these people and you're going to have to have planes to fly them out. So, again, you could eventually deport a lot of people this way, but it's not going to all happen on day one of the presidency."