How Can Trump Deploy the Military at the Southern Border?

On Nov. 18, President-elect Trump endorsed “declar[ing] a national emergency” in order to “use military assets” for “a mass deportation program.” In truth, there is relatively little useful legal authority that Trump can unlock by invoking the National Emergencies Act (NEA) to use the military at the southern border. And what legal authority he can make available will not enable an immediate, comprehensive plan for mass deportations. This does not mean, however, that Trump will be unable to use the military for mass deportations. Using the playbook developed during his first term in office—and legal practices that extend across party lines—there is far more that Trump can achieve outside the NEA context. 

National Emergency and Military Personnel Deployments

It is important to remember that under the current administration, there is already a national emergency being used for deployments of the military to the southern border. This is not to suggest that Trump’s border policies are equivalent to Biden’s, but to reinforce that the legal framework at issue here is far from new. 

As I have described previously, in December 2021 President Biden invoked the NEA on the grounds that “international drug trafficking ... constitutes an unusual and extraordinary threat to the national security, foreign policy, and economy of the United States.” In April 2023, Biden amended this declaration to allow the Defense Department to order members of the reserve components (including National Guard) to active duty for up to two years. Pursuant to this declaration, in 2023 the Defense Department deployed 1,500 National Guard personnel to the southern border to support the Department of Homeland Security. In December 2023, Biden renewed this emergency for another year. Currently, there are 2,500 National Guard personnel supporting Homeland Security at the southern border under authorities made available by this NEA declaration.

I would expect Trump’s southern border national emergency to continue this authority to more quickly bring National Guard personnel into an active-duty status. He did, after all, do the same during his first term in office. But I would be surprised if the Trump Defense Department makes much use of the authority (other than by continuing the existing National Guard deployment). This is because Trump will be able to achieve many more of his mass deportation aims with the military if he deploys National Guard personnel pursuant to another, non-emergency, statutory authority—32 U.S.C. § 502(f).

Section 502(f) authorizes the president or secretary of defense to request that governors provide members of their National Guard to perform “other duty” in support of a federal military mission. The signal benefit of this duty status is that these National Guard personnel remain part of their state and territorial militias and, therefore, are not subject to the Posse Comitatus Act (PCA). This is important because the PCA prohibits active-duty military personnel from performing law enforcement functions.

Being unconstrained by a prohibition against conducting law enforcement functions is not the same, however, as authorizing the military to perform those functions. But this is not a significant obstacle, particularly at the southern border and even without emergency authority. Section 1059 of the National Defense Authorization Act for Fiscal Year 2016 (10 U.S.C. § 284, note) authorizes the secretary of defense to provide assistance to U.S. Customs and Border Protection “for the purposes of increasing ongoing efforts to secure the southern land border.” The statute doesn’t limit the forms of support the Defense Department may provide. Instead, it provides that the assistance “may include” military personnel deployments to the southern border, deployment of manned and unmanned aircraft and ground-based surveillance, and intelligence analysis support.

President Trump, in his first term, already used National Guard personnel in a § 502(f) duty status to perform immigration enforcement duties under § 1059. And while the duties they performed were relatively limited, there is nothing in the text of § 1059 that would limit future deployments from including a far wider range of law enforcement actions.

While the alchemy of § 1059 and § 502(f) is unique to the Trump administration, since 2001 presidents of both parties have used § 502(f) to get around the PCA’s prohibitions against the military being used for law enforcement. After Sept. 11, 2001, for example, the Bush administration used this duty status to enable the National Guard to provide security at the nation’s airports. Bush again deployed the National Guard in a § 502(f) status to the southern border and New Orleans (the latter after Hurricane Katrina). President Obama did the same for his own deployment of National Guard personnel to the southern border. Indeed, the longest use of § 502(f) ended only in the early days of the Biden administration, after tens of thousands of National Guard personnel were deployed throughout the country to perform a huge range of functions in response to the COVID-19 pandemic.

The only meaningful limitation to a § 502(f) deployment that the executive branch has recognized is one of fiscal law. Ordinarily, 10 U.S.C. § 277(c) provides that law enforcement agencies reimburse the Defense Department for National Guard support provided in a § 502(f) duty status. However, the secretary of defense may waive this requirement if the support is provided “in the normal course of military training or operations” or if it results in a benefit for the members involved that is “substantially equivalent to that which would otherwise be obtained from military operations or training.” The current chief of the National Guard Bureau has testified that these southern border deployments provide no training value. I doubt, however, that this will be much of an impediment to executive action. During the Trump administration, multiple secretaries of defense waived this requirement for all National Guard southern border deployments. Multiple secretaries made the same determination for the many thousands of National Guard personnel deployed to the Capitol after the Jan. 6 attack. I have not found congressional objection to either of these determinations.

The NEA and these mobilization authorities are distinct from the Insurrection Act, a separate set of legal authorities with a separate mobilization provision authorizing the president to use the military to respond to domestic unrest. There has been widespread speculation about whether President-elect Trump will invoke the Insurrection Act for southern border deployments, as proposed in Project 2025 documents. The fact that he did not mention this authority in his Nov. 18 post on Truth Social, however, is cold comfort. Whereas presidents must first issue a public proclamation to disperse under the Insurrection Act, there is no such transparency requirement for a military deployment under § 1059 (though there is a congressional reporting requirement). So, while we would expect to know the details and legal contours of a southern border deployment under the Insurrection Act, there is no guarantee of the same transparency under the non-emergency statutory powers on which Trump is more likely to rely. 

National Emergency and Migrant Detention Camps

In the end, there is likely only one important authority that Trump will make available under a southern border national emergency—10 U.S.C. § 2808. Section 2808 authorizes the secretary of defense “without regard to any other provision of law, [to] undertake military construction projects ... not otherwise authorized by law that are necessary to support such use of the armed forces.” Military construction has a broad definition: “[A]ny construction, development, conversion, or extension of any kind carried out with respect to a military installation, whether to satisfy temporary or permanent requirements, or any acquisition of land.” A military installation is further defined as “a base, camp, post, station, yard, center, or other activity under the jurisdiction of the Secretary of a military department.” If this legal framework rings a bell, it should. President Trump invoked this same authority during his first administration to build a border wall on military installations. 

The New York Times has reported that a singular focus of President Trump’s forthcoming immigration plan is expanding the federal government’s capacity to detain migrants for deportation. I expect to see the Trump administration use § 2808 to authorize the construction of migrant detention camps on military installations. This wouldn’t be the first time that a significant number of civilians, indeed a significant number of migrants, would be detained on military bases. During the earliest days of the COVID-19 pandemic, for example, the Defense Department housed many hundreds of evacuees from China for quarantines. After the fall of Kabul, the Defense Department housed tens of thousands of evacuees across eight domestic military installations. And in 1980, many thousands of Cubans were housed in military installations after fleeing to South Florida (the Mariel boatlift). 

What’s more, the Department of Justice Office of Legal Counsel (OLC) has long held that military personnel may take a variety of law enforcement actions to maintain law and order among civilians on a base, notwithstanding the PCA. Regarding the Mariel boatlift, in 1980 OLC wrote that “[m]ilitary personnel may apprehend and restrain parolees for on-base violations of federal and state law which in the base commander’s view threaten the security and good order of the base.” This included the use of “necessary force against civilian conduct threatening military equipment or facilities provided for the use” of those on the base. It further included the ability to “patrol within” the area reserved for civilians. These conclusions were, in turn, based on a similar 1977 OLC opinion, which relied on U.S. Court of Appeals for the Ninth Circuit precedent to find that the PCA “does not prohibit military personnel from acting upon base violations committed by civilians.” 

These examples of humanitarian housing are not equivalent to the nationwide program to round up, detain, and deport migrants that is coming into view. But it does once again show the considerable precedent on which the Trump administration will be able to rely in piecing together its mass detention program. Without a doubt, the aspect of this plan that is most susceptible to litigation is using § 2808 authority to build temporary or permanent structures to house migrants. We saw similar litigation during the first Trump administration, objecting to his use of § 2808 for border wall construction. Again, this history is instructive.

In the border wall context, the Defense Department incorporated new areas under the jurisdiction of existing military installations on which it planned to build a border wall under § 2808. The Ninth Circuit upheld a lower court decision finding that these additions to existing military installations were not truly a military installation, as they did not truly act as a military base, camp, station, yard, or center. The Ninth Circuit further held that the border wall did not support a Defense Department mission, distinguishing truly military missions from immigration support provided to the Department of Homeland Security. 

A plan to build temporary migrant detention centers on military installations will encounter some, but not all, of these same hurdles. First, it seems less immediately necessary for the Defense Department to incorporate new land into a military installation to build a detention facility. So the first objection to using § 2808 is unlikely to be relevant. Whether Defense Department support to the Department of Homeland Security, or immigration enforcement writ large, is sufficient to constitute military construction, however, will be a live issue. Importantly, it is also a question on which there is not formally any binding precedent in any jurisdiction, the Ninth Circuit included. In 2021, at the Biden administration’s request, the Supreme Court vacated both the Ninth Circuit’s judgment and that of the district court.

***

It is possible to project, with a considerable degree of confidence, what Trump will (and will not) be able to achieve with the military at the southern border. There is ample non-emergency authority on which he can rely to deploy the military for a range of law enforcement duties at the southern border. A NEA declaration will be important, however, for any use of Defense Department funds to establish migrant detention camps on military installations. We should expect these efforts to be litigated in court over the course of many years, if history is any guide. 

Which leads to my final point. There is nothing about any of the actions described here that is immediate. A National Guard deployment under § 502(f) and § 1059 takes time to develop. The Defense Department has to understand which governors have available National Guard personnel and are willing to provide those personnel for federal duty in a § 502(f) duty status. The Department of Homeland Security must request Defense Department support under § 1059. The military construction process is notoriously slow and cumbersome (though § 2808 does considerable work to expedite this process). There will be a barrage of executive actions on Jan. 20. We must carefully distinguish which pose immediate threats, and which are political theater.