According to the New York Times, Trump is planning to invoke the Insurrection Act to bring in the military to carry out mass deportations. The law is a key exception to the Posse Comitatus Act, which prohibits the use of the military to enforce federal law without the permission of Congress or the Constitution.

Only in rare instances have presidents invoked the Insurrection Act. President George H.W. Bush was the last one to do so amid the 1992 Los Angeles riots that broke out in response to the acquittal of police officers in the beating of Rodney King. President Dwight D. Eisenhower also notably used the Insurrection Act to facilitate the desegregation of schools in Little Rock, Arkansas.

The provision of the Insurrection Act most likely to apply in Trump’s case is one that allows the president to unilaterally activate the military domestically to enforce federal law whenever they determine that “unlawful obstructions, combinations, or assemblages, or rebellion… make it impracticable [to do so] by the ordinary course of judicial proceedings.”

Mirasola said Trump would have a “relatively easy time” making the case that cartels trafficking immigrants across the border constitute an “unlawful obstruction” to the enforcement of US immigration law. Trump has in some ways appeared to begin building his case for invoking the Insurrection Act through his rhetoric on the campaign trail this year by describing an “invasion of criminals” coming across the border.

But Mirasola said it would be harder for Trump to argue that it is impracticable to enforce immigration laws through the “ordinary course of judicial proceedings.” That’s because presidents have done so for decades, and border crossings are no longer unusually high: They have sharply declined this year and are down even from certain points in the first Trump administration.

However, the law gives the president “sole discretion, in most instances” to determine whether the criteria necessary to activate the military have been met, according to 2022 congressional testimony given by Elizabeth Goitein, co-director of the liberty and national security program at the Brennan Center for Justice, and Joseph Nunn, the Center’s counsel in the national security program.

Goitein and Nunn also argued that the “vague and broad criteria for invoking the Act, combined with the lack of any provision for judicial or congressional review, render it ripe for abuse.” At that point, their concern was that Trump could have used the Insurrection Act to interfere with the certification of the 2020 election results. The use case is now different, but the potential for overreach is the same.

That is to say, while advocates may challenge Trump on whether the two key criteria for invoking the law have been met, the law gives presidents a wide berth — and the courts little power.

“For all practical purposes, courts have been cut out of the process,” Goitein and Nunn write.

There are other potential authorities that Trump could invoke to surge military resources to his mass deportation plan.

As Mirasola writes in Lawfare, Trump has a nonemergency power under federal law to request the assistance of state national guards in a federal military mission. Under the National Defense Authorization Act, that mission can be to assist US Customs and Border Protection in “ongoing efforts to secure the southern land border.” The law does not provide parameters limiting the kind of assistance that the military can provide, be that boots on the ground at the border or intelligence analysis support.

Emergency powers could be helpful in creating the infrastructure needed for mass deportations. Stephen Miller, one of Trump’s key immigration advisers, told the New York Times in November 2023 that a second Trump administration would construct “vast holding facilities that would function as staging centers” for immigrants facing deportation. Mirasola writes that, to do so, Trump could invoke federal law allowing the secretary of defense to “undertake military construction projects … not otherwise authorized by law that are necessary to support” the armed forces in a national emergency.

If Trump declares a national emergency with respect to immigration, that law would essentially allow him to bypass the need for congressional approval to get the funds he needs to construct these holding facilities. He previously used the same law to try to get funding for his border wall during his first term. Whether he could do so was never settled.

Pro-immigration advocates challenged the use of that law to fund the border wall in Trump’s first term. Their years-long litigation over the border wall became moot when President Joe Biden took office, but they were not expected to win if the issue had come before the Supreme Court. Advocates could again mount a legal challenge, but they may only succeed in delaying the construction of the facilities.

However, pro-immigration advocates might have a stronger case if they file lawsuits over the conditions in these yet-to-be built holding facilities and over potential violations of civil liberties for immigrants subject to mass deportations. Those might involve, for example, violations of their constitutional right to due process. That sort of challenge, over inhumane detention conditions previously seen in CBP facilities (including a lack of access to basic hygiene products and a lack of food, water, and basic medical care) was successfully made during the first Trump administration.

Immigrants might also file suits arguing their constitutional protections against unlawful searches were violated: Doris Meissner, senior fellow and director of the US Immigration Policy Program at the Migration Policy Institute, said mass deportations of the scale Trump is imagining would likely involve “violations of people’s civil rights, profiling, all of those kinds of harms that poor policing brings about.”

That will present a key test for the courts, Michael Waldman, president and CEO of the Brennan Center for Justice at NYU School of Law, said in a statement: “Will [the courts] use their power to enforce long-standing protections for individuals? Will they uphold the rule of law? Or will they bow to political pressure and allow the executive to expand its already ample power?”