A cornerstone of former President Donald Trump’s 2024 campaign has been his promise to carry out the largest deportation operation in United States history. The details of how he would carry out the plan have been unclear. But at recent rallies, Trump has said he will use an 18th-century law to enforce mass deportations.

The deportation operation will begin in Aurora, Colorado, and will be called “Operation Aurora”, Trump said at an October 11 rally in Reno, Nevada, adding that immigrants are “trying to conquer us”.

Earlier that day at a campaign rally in Aurora, he said he’d invoke the Alien Enemies Act of 1798 to expedite gang members’ removal and to “target and dismantle every migrant criminal network operating on American soil”.

Trump was referring to a Venezuelan gang, Tren de Aragua, which he said has taken over “multiple apartment complexes” in Aurora. Claims that a Venezuelan gang had taken over Aurora started in August, when a video of a group of Spanish-speaking armed men walking in a city apartment complex went viral. However, local officials have pushed back, saying that concerns about Venezuelan gangs in Aurora are “grossly exaggerated”.

Aurora police say they’ve arrested Tren de Aragua gang members, but they have not said they had taken over apartment complexes.

Here’s what we know about the 1798 law Trump promised to invoke and what legal experts say about Trump’s ability to use it for mass deportations.

What is the Alien Enemies Act of 1798?

The Alien Enemies Act of 1798 is part of a larger set of four laws – the Alien and Sedition Acts – that the United States passed as it feared an impending war with France. The laws increased citizenship requirements, criminalised statements critical of the government and gave the president additional powers to deport noncitizens.

Three of the laws were repealed or expired. The Alien Enemies Act is the only one still in place.

The law lets the president detain and deport people from a “hostile nation or government” without a hearing when the US is either at war with that foreign country or the foreign country has “perpetrated, attempted, or threatened” an invasion or raid legally called a “predatory incursion” against the US.

“Although the law was enacted to prevent foreign espionage and sabotage in wartime, it can be – and has been – wielded against immigrants who have done nothing wrong” and who are legally in the US, Katherine Yon Ebright, an expert on constitutional war powers at the Brennan Center for Justice, a nonpartisan think tank at New York University School of Law, wrote in an October 9 report for the Brennan Center for Justice.

The law was last invoked during World War II

US presidents have invoked the law three times, only during wartime:

• The War of 1812: Former President James Madison invoked the act against British people who were required to report information including their age, the length of time they’d lived in the US and whether they’d applied for citizenship.
• World War I: Former President Woodrow Wilson invoked the act against people from Germany and its allies, such as Austria-Hungary.
• World War II: Former President Franklin Roosevelt invoked the act “to detain allegedly potentially dangerous enemy aliens”, the National Archives said. Mainly this included German, Italian and Japanese people. The act was used to place noncitizens from those countries in internment camps. The act was not used to detain US citizens of Japanese descent. An executive order was used for that.

Can Trump use the act to carry out mass deportations?

Trump has mentioned enforcing the 1798 law against Mexican drug cartels and Tren de Aragua, the Venezuelan gang.

Legal experts said Trump does not have the authority to invoke the Alien Enemies Act against gang members or as a tool for mass deportations.

To invoke the act, an invasion must be perpetrated or threatened by a foreign government. The US is not currently at war with any foreign government. The law also can’t be used broadly for people from every country.

Invoking the act “as a turbocharged deportation authority … is at odds with centuries of legislative, presidential, and judicial practice, all of which confirm that the Alien Enemies Act is a wartime authority”, Ebright said in her report. “Invoking it in peacetime to bypass conventional immigration law would be a staggering abuse.”

Trump and his allies have characterised the rise in illegal immigration under President Joe Biden as an invasion. Legal and immigration experts have disagreed with the characterisation.

The illegal migration or drug smuggling at the southern border is not an invasion, Ilya Somin, a George Mason University constitutional law professor wrote in an October 13 report for online magazine Reason.

Legal experts have said that an attempt to use the Alien Enemies Act for mass deportations would likely be challenged in court. However, it’s unclear whether the courts would issue a ruling.

A court last heard a case regarding the Alien Enemies Act after World War II. Former President Harry Truman had continued Roosevelt’s invocation of the act for years after the war’s end. At the time, the court ruled that whether a war ended and whether wartime authorities had expired were “political questions” and therefore not up to courts to decide.

Similarly, some courts have previously said that the definition of an invasion is also a political question.

Trump has previously promised mass deportations.

During his 2016 presidential campaign, Trump promised to deport all immigrants living in the US illegally. However, he failed to do this.

When Trump entered office, an estimated 11 million people were illegally in the country, according to data from Pew Research. From fiscal years 2017 to 2020, the Department of Homeland Security recorded 2 million deportations. (Fiscal year 2017 included about four months of former President Barack Obama’s administration.) By comparison, Obama carried out 3.2 million and 2.1 million deportations during each of his terms respectively.

The Migration Policy Institute, a nonpartisan think tank, reported in June that the Biden administration has carried out 4.4 million deportations, “more than any single presidential term since the George W Bush administration (5 million in its second term)”.

Steve Vladeck, a Georgetown University constitutional law professor, wrote in his newsletter on October 14 that there are already immigration laws that allow for deportations. But a main challenge against carrying out a mass deportation operation is the lack of resources required to find, detain and deport a large number of people.

“Relying on an old statute won’t help solve the resources problem,” Vladeck said. ​