Former President Donald Trump has made border security the cornerstone of his reelection campaign — and if he wins in November, he will seek to implement a slate of new policies to discourage the record-high border crossings of recent years.

Some of those policies, according to his campaign platform, could involve the U.S. military.

If elected, Trump says he would “use all resources needed to stop the Invasion — including moving thousands of Troops currently stationed overseas to our own Southern Border.”

It’s not yet clear whether Trump is referring to National Guard deployments to border regions — an action presidents of both parties have taken to help U.S. Customs and Border Protection when border crossings are high — or whether he would seek to shift active-duty troops currently stationed overseas to more homeland security-focused roles, which would be a more significant policy change.

The Trump campaign has also vowed to use the U.S. Navy to combat fentanyl smuggling and mobilize military assets against Mexican drug cartels.

To Trump’s supporters, the proposals are part of a broader shift to an “America First” foreign policy that prioritizes protecting the homeland over involvement in foreign conflicts. They’re also a necessary step to combat illegal drug trafficking that has led to tens of thousands of American deaths.

“President Trump will prioritize — I think he will always put America first,” said Tom Homan, who served as acting director of Immigrations and Customs Enforcement during the last Trump administration and is now a fellow at the Heritage Foundation, a conservative think tank with ties to Trump. “It doesn’t mean you pull assets out of Ukraine or you pull assets out of other places in the world. I think we have plenty of assets go around, but if it requires a redeployment of assets to protect this country, then it absolutely needs to be done.”

National Guard deployments

If elected, Trump will likely continue deployments of National Guard troops and even small numbers of active-duty troops to the border — a step both Democratic and Republican presidents have taken to augment the Department of Homeland Security’s resources in the area.

Trump’s Defense Department in 2018 authorized up to 4,000 National Guard troops to support the Homeland Security Department’s border mission amid a “drastic surge of illegal activity on the southern border.” Later that year, the administration dispatched up to 800 active-duty troops to assist the National Guard forces already present.

And President Joe Biden in 2023 announced plans to send 1,500 active-duty troops to the border on top of 2,500 National Guard troops already deployed at the time, in anticipation of the rollback of pandemic-era border restrictions.

DOD personnel deployed to the border do not perform law enforcement functions, instead fulfilling administrative roles to free up CBP agents for fieldwork. Active-duty servicemembers cannot perform law enforcement duties inside the U.S. unless specifically authorized by Congress.

“So what DOD can do, the military can do, is do those non-law enforcement duties,” Homan said. “Take those duties away from the Border Patrol, which puts more law enforcement officers doing exactly what they should be doing — law enforcement and arrest and detention.”

DOD involvement at the border in support of DHS has been common in recent administrations, but if Trump seeks to shift large numbers of active troops from overseas roles to the border, that would be more controversial, said Carrie Cordero, a senior fellow at the Center for a New American Security who previously served at the Homeland Security Advisory Council.

“If the concept is to withdraw active-duty military who are deployed overseas on military assignments and assign non-National Guard, but full-time active military to the southern border, then that would be a completely different issue,” she said.

She noted there are foreign policy implications of pulling troops from overseas duty as well as legal complications of deploying large numbers of active-duty troops for civil law enforcement purposes.

However, Katherine Kuzminski, director of the Military, Veterans, and Society Program at CNAS, said “it is far more likely that Trump would seek to deploy National Guard troops to the U.S.-Mexico border on a smaller scale than it would be to move active-duty troops from overseas roles.

“I think the most likely scenario is the use of the Guard — not necessarily moving troops from overseas,” she said. “But I think that it plays well politically for him to say that he’s going to bring troops home from another country, because it fits into a broader non-interventionist policy portfolio.”

Combating cartels

Trump’s other policy proposals involving the military in border security have less precedent, experts say, and could come with additional challenges.

Using the U.S. Navy to interdict fentanyl trafficking would complicate efforts already underway by the U.S. Coast Guard, which typically has responsibility for those activities.

“The Navy can call up the Coast Guard to Title 10 authority in a wartime footing, but you don’t normally see the responsibilities going the other way, where the Navy is supporting the Coast Guard,” Kuzminski said.

And militarily targeting criminal organizations responsible for drug smuggling could threaten the U.S. diplomatic and economic relationship with Mexico or pull resources from other counterterrorism missions.

“In terms of the economic relationship with Mexico and the nature of the relationship between the countries, treating the cartels as terrorist organizations would be a major, major change in how the U.S. does policy,” Cordero said.

However, using military resources to go after cartels isn’t a new idea, and there is some congressional support for it. In 2023, Reps. Daniel Crenshaw, R-Texas, and Michael Waltz, R-Fla., introduced legislation that would authorize the use of military force to target Mexican drug cartels “facilitating the fentanyl crisis.”

“We cannot allow heavily armed and deadly cartels to destabilize Mexico and import people and drugs into the United States,” Crenshaw said in a statement at the time. “We must start treating them like ISIS — because that is who they are.”

Other Republican candidates for elected office in recent years have echoed those calls; Florida Gov. Ron DeSantis during his presidential primary campaign vowed to send special operations forces into Mexico to combat drug cartels.

“Bottom line is, the Mexican cartels in Mexico have killed more Americans than every terrorist organization in the world combined,” Homan said. “They are a threat to our national security. There are clear danger to this country.”

Congressional approach

Congressional Republicans more broadly are increasingly eyeing the Defense Department’s role in protecting the U.S.-Mexico border. The Senate version of the fiscal 2025 defense authorization bill — which advanced out of committee on a bipartisan basis in June — contains several provisions that would deploy Pentagon resources for certain border security activities.

The bill would promote information-sharing between the Defense Department and federal, state and local authorities deployed to the southwest border, authorize the Pentagon to enter services contracts in support of CBP, and allocate an additional $25 million for Joint Task Force North to counter transnational criminal organizations in the region.

“The effort to control the border rightly needs to be led by the Department of Homeland Security, but the Department of Defense can deploy assets and assistance to support DHS when necessary,” said Sen. Roger Wicker, R-Miss., ranking member of the Senate Armed Services Committee, in a statement. “DOD should also offer up select assets for intelligence and information-sharing, deploy certain DOD personnel, and strengthen coordination with other federal agencies.”