President-elect Trump confirmed his intent to use the US military for mass deportations in the US.

Laws sharply limit the roles that federal troops can fill in US law enforcement.

Trump could rely on state-led National Guard personnel or attempt to bypass a long-standing law.

President-elect Donald Trump said this week that his incoming administration plans to follow through on his campaign promise of using the US military to execute his mass deportation plan.

This puts his incoming administration on a potential collision course with long-standing laws and practices that sharply limit the use of US troops in law enforcement.

Trump's border czar has said US troops could assist immigration dragnets through non-enforcement roles that involve building structures, gathering intelligence, or flying migrants to the countries they fled. National Guard troops that report to state governors can support law enforcement, but the Posse Comitatus Act bars active-duty and federalized Guardsmen from acting as law enforcement.

Trump could rely on state-led National Guardsmen, however, or attempt to sidestep Posse Comitatus altogether through the Insurrection Act of 1807. But this would be an extreme move that's sure to trigger fierce opposition from state governors, lawsuits, and military officials.

"If the president were looking to use the Instruction Act to enforce immigration as a federal law, that would be a pretty extraordinary use, and that would be way out of custom," Mark Nevitt, who served as a judge advocate general in the US Navy, told Business Insider. "Arguably, there is an authority to do that, but again, it would be way out of the norms of its historic use."

When asked how the incoming administration would confront these limitations, Karoline Leavitt, a spokeswoman for Trump's transition team, said the president-elect "will marshal every federal and state power necessary to institute the largest deportation operation" in US history.

"The American people re-elected President Trump by a resounding margin giving him a mandate to implement the promises he made on the campaign trail, like deporting migrant criminals and restoring our economic greatness," Leavitt told Business Insider in an emailed statement. "He will deliver."

What happens if Trump orders the US military to assist?

Each military branch has a civilian general counsel, who would likely be among the first officials to review whether a presidential order complies with the Posse Comitatus Act, said Gary Solis, a former Marine JAG. He described two potential scenarios:

The Army's general counsel concludes it is lawful, allowing US armed forces to move forward with Trump's order. This scenario would likely trigger a slew of lawsuits attempting to block its enforcement, Solis said.

The general counsel rejects the order, giving lower-level commanders the grounds to refuse it.

"But no law can interpret a presidential order in advance," Solis said. "Some orders would be obviously unlawful, but any order issued from Trump's office would be very carefully worded in an effort to make its execution by the military bulletproof."

Nevitt, an associate law professor at Emory University, said the PCA provision is "really, really nuanced" and doesn't explicitly "prohibit the National Guard from enforcing immigration laws."

The president can legally deploy the National Guard for domestic law enforcement through cooperation with state governors, provided that the military force is operating under state control and not federalized through the Insurrection Act and thus subjected to PCA restrictions.

"You can imagine that some governors will be more excited about this mission than others," Nevitt told BI. "You can fill in the blank on who those might be, but probably more Republican governors that are more friendly to President Trump. Those who are less friendly to President Trump would maybe not be as interested in taking on this mission."

"If President Trump tried to use this authority from a red state like Wyoming or Texas and put those National Guard troops in a blue state that is unwilling to accept them, that would be quite a crisis," he continued. "I think the state that did not want to accept these outside National Guard troops could say this is a violation of their own sovereignty, and they'd have a pretty powerful case that they could find a way to a lawsuit."

What roles can the US military fill?

On his social media platform, Truth Social, the former president this week commented "TRUE!!!" on a post about his plans to "declare a national emergency" and "use military assets" to carry out sweeping raids to deport millions of migrants a year.

Related stories

Declaring a national emergency can provide the president with a broader set of powers to respond to crises, but it doesn't authorize the military to act as domestic law enforcers. The Trump administration may not attempt to use them as such to avoid triggering a backlash.

Agencies like US Immigration and Customs Enforcement lead efforts to locate and arrest migrants. A more robust immigration crackdown could require building bigger detention centers, where migrants are held through the deportation process. That's where Defense Department funding and personnel could come in.

Trump's "border czar," Thomas Homan, said using military funds would be a "force multiplier" in an immigration crackdown; Trump's first administration redirected billions of dollars from the Pentagon to build sections of the wall on the US-Mexico border.

But Homan, who formerly led ICE, specified that military personnel would be assigned "non-enforcement duties, such as transportation, whether it's on ground or air, infrastructure, building, [and] intelligence."

"We're hoping DoD will help us with air flights because there's a limited number of planes ICE has contracts with, so DoD can certainly help with air flights all across the globe," Homan said in a Tuesday interview on Fox Business Network.

Will troops want to become police?

Aside from the many legal challenges, Nevitt said he thinks that the troops might be reluctant to participate on the stance they signed up to be warfighters — not policemen.

"The military has, historically, not wanted this mission," Nevitt, who served in the Navy for two decades, said. "The federal military forces want to fight and win our nation's wars; they want to secure the nation's national security; they want to do operational deployments."

"In asking federal military forces to enforce immigration laws, there is going to be a strong cultural allergic reaction that's well grounded in civil-military norms," he added.

Nevitt said he thought federal troops carrying out domestic law enforcement would tarnish the "special trust" the American public holds in the military.

"As a veteran, the military is seen as protecting this country, keeping our country safe," he said. "There's going to be a lot of static if President Trump asks the military to do something that is beyond what they have historically been asked to do."