Editor’s note: Approximately 1,500 active duty service members will arrive at the southern US border this week as Title 42, a measure put in place during the COVID-19 pandemic that allowed rapid expulsion of certain migrants, is set to expire, prompting expectations of increased border crossings. President Joe Biden’s administration has said these forces will serve in administrative roles, but what does the law allow active duty military personnel to do on US territory? And how does that differ from what National Guard members—of which there are currently 2,500 on the border—are legally able to do? This primer, a version of which was previously published in 2018 when the previous administration deployed active duty personnel to the border, describes the legal and policy framework governing the deployment of US military forces on American territory.

What can the military do at the border? And what can’t it do? Examining the specific legal and policy issues at the center of the debate yields a conclusion many readers might be surprised by.

No discussion can address these questions adequately without first making clear what some of the terminology involved means. For example, not all “militaries” are equal—at least in terms of the law. The National Guard is a state military asset first in that it serves under the command authority of the state governor in both State Active Duty (SAD) and Title 32 status. In past National Guard troop deployments to the border, Guard forces served in SAD status in most instances. Whereas the Guard serves as a state asset in both SAD and Title 32 status, “when called into the actual service of the United States” National Guard troops serve as “federalized” military forces under Title 10 United States Code (USC). In Title 10 status, the Guard serves under the command of the president and functions as part of the Armed Forces of the United States, which includes the active and reserve components of the Army, Navy, Air Force, Marine Corps, and Space Force. The president is the commander-in-chief of the Armed Forces of the United States in all circumstances, even domestic border support.

What Can the Armed Forces Do at the Border?

When the armed forces are called upon to operate on US soil but beyond the confines of their bases and installations, limitations apply. The Department of Defense provides defense support of civil authorities (DSCA) when requested by a civil authority and approved through a formal process outlined in DoD policy. DSCA takes on many forms including support for domestic emergencies and disasters, law enforcement support, and support for special security events. The following discussion on DSCA only applies to the armed forces and does not apply to the National Guard unless federalized under Title 10 status as described above.

Defense Support of Civil Authorities (DSCA)

In response to requests from civil authorities, the armed forces can support state and local governments in accordance with law and DoD policy. Military operations under DSCA must be formally approved in most cases before the military can intervene. However, under immediate response authority outlined in DSCA policy, DoD can provide immediate—albeit time-limited— support absent requests from civil authorities during no-notice or limited-notice incidents in order to “save lives, prevent human suffering, or mitigate great property damage.” Other operational requests such as support to special security events require formal approval processes but still fall under the DSCA purview (for more on defense support of special events, see DoD Instruction 3025.20). Military forces providing DSCA often interact with law enforcement, especially during incident response or high-profile security events. Given the sensitive nature of military involvement in law enforcement domestically, DoD Instruction 3025.21—in conjunction with Chapter 15 of Title 10 USC—establishes the guidance for what the armed forces can do in support of civilian law enforcement.

Defense Support of Civilian Law Enforcement (DSCLE)

In DSCLE, the armed forces provide law enforcement support in accordance with law and DoD policy and when requested by a civilian law enforcement entity. When supporting law enforcement, DoD actions are generally limited to:

  • conducting investigations;
  • protecting DoD personnel and equipment;
  • securing classified material;
  • actions that further the DoD or foreign affairs interests of the United States;
  • operation and maintenance of equipment under specific circumstances;
  • transportation of personnel; and
  • training

Despite the ostensibly clear guidelines, this doesn’t mean military personnel have not in the past been involved in other activities. In 2018, President Donald Trump deployed forces to the border, and although the words “fencing,” “border,” “barrier,” “perimeter,” and even “wall” appear nowhere in the permissible military activities above, the US Army was involved in building concertina wire fencing to reinforce existing border infrastructure. Why? DoD Instruction 3025.21 is ripe with ambiguities allowing for broad interpretations of what constitutes permissible support. Consider that after prescribing the activities above the document then adds to this list “such other actions that are undertaken primarily for a military or foreign affairs purpose” and “such activities that are necessary to prevent significant loss of life or wanton destruction of property and are necessary to restore governmental function and public order.” This provides an open door, so to speak, for use of military forces to fulfill myriad roles.

Critics will say the military shouldn’t perform any function that isn’t expressly authorized by law or policy. However, it is impractical to codify in written policy every conceivable military function in support of law enforcement. Such nebulous guidance is necessary so as not to restrict support functions to only those meeting detailed and prescriptive requirements. Despite the vague guidance, the instruction specifically prohibits the armed forces from performing traditional law enforcement activities.

What Can’t the Armed Forces Do at the Border?

Save for express constitutional authorizations or acts of Congress (which I will get to shortly), the US military can’t serve as another police force arresting migrants on site. The Posse Comitatus Act (PCA) prohibits the deliberate use of the armed forces to enforce law on US soil and is an often-referenced law in border support discussions. The PCA does not apply to the US Coast Guard as Congress granted the Coast Guard law enforcement functions under Title 14 USC. Superficially, the armed forces can’t execute law domestically. The question is what exactly constitutes “executing law.” According to DoD policy—as informed by federal law—the armed forces are prohibited from performing the following law enforcement activities:

  • interdiction of a vehicle, vessel, aircraft, or other similar activity;
  • a search or seizure;
  • an arrest; apprehension; stop and frisk; engaging in interviews, interrogations, canvassing, or questioning of potential witnesses or suspects; or similar activity;
  • using force or physical violence, brandishing a weapon, discharging or using a weapon, or threatening to discharge or use a weapon except in self-defense, in defense of other DoD persons in the vicinity, or in defense of non-DoD persons, including civilian law enforcement personnel, in the vicinity when directly related to an assigned activity or mission;
  • evidence collection; security functions; crowd and traffic control; and operating, manning, or staffing checkpoints;
  • surveillance or pursuit of individuals, vehicles, items, transactions, or physical locations, or acting as undercover agents, informants, investigators, or interrogators; and
  • forensic investigations or other testing of evidence obtained from a suspect for use in a civilian law enforcement investigation

Despite this and other policy language, ambiguity remains. The messaging justifying the upcoming use of federal troops on the border is the important point. Is the border mission civil support? Or is it homeland defense in response to a threat? Will troops be carrying weapons and able to act in self-defense? Or will they truly serve in exclusively administrative roles as the president suggests? Words matter.

Take for instance the 2019 incident in which a US Marine at a “mobile surveillance camera site” near the US-Mexico border fired his weapon in self-defense, claiming he was attacked. At the time, officials insisted the military was there in a strictly support role and not conducting law enforcement activities along the border. The problem? DSCA policy prohibits military personnel from carrying, brandishing, and discharging weapons during civil support missions, unless authorized by the secretary of defense under specific use-of-force parameters permitting self-defense. Moreover, DSCLE policy prohibits military personnel from conducting surveillance. Yet in this mission ostensibly in support to civil authorities and law enforcement—where neither using weapons nor surveillance activities were permissible for military personnel—an active duty US Marine was conducting armed surveillance. So, what gives?

US troops serving in remote border posts should maintain their right to self-defense consistent with use-of-force guidelines. Neither the weapon nor the surveillance was the issue in this case. The Trump administration’s use of troops on the border was clear: military troops were there to “protect the homeland.” As a homeland defense mission, carrying weapons and conducting surveillance are fair game for US military personnel. Ironically, other government officials obfuscated this issue through inconsistent and contradictory messaging seeking to appease public discourse. This was four years ago and is an interesting anecdote considering the recent news of a new round of troop deployments for a seemingly similar, yet thus far undefined, purpose.

What Constitutes a Threat?

An important caveat to all of this is that if the president or Congress deems there to be a threat to the United States, there are exceptions to the Posse Comitatus Act that would legally permit the armed forces to take military action and perform law enforcement functions to defend the homeland from the perceived threat. The crux of this issue is principally an interpretation of what constitutes a threat or aggression to the US sovereign and its people.

Unlike homeland security—which emphasizes prevention of terrorist attacks and in which the armed forces have no direct role—homeland defense is “the protection of United States sovereignty, territory, domestic population, and critical infrastructure against external threats and aggression or other threats as directed by the President,” and DoD is the lead federal agency in such situations. In assessing threats, homeland defense doctrine focuses on coordinated Red Dawn-esque attacks. But it also specifically addresses transnational threats—and includes “ongoing illegal immigration” as activity that meets the definition of a transnational threat. Further, homeland defense is considered a “constitutional exception to the PCA” such that the armed forces performing designated homeland defense operations are not subjected to the restrictions of the Posse Comitatus Act, which seemingly justified the Marine’s actions in the 2019 situation mentioned above.

But this isn’t the only exception to PCA restrictions. The Insurrection Act—like homeland defense—is a constitutional exception to the Posse Comitatus Act that permits the president to call into federal service both the militia (National Guard) and the armed forces to enforce laws or suppress an insurrection, a rebellion, or domestic violence in the United States. Beyond the Insurrection Act, the Calling Forth Act (Article I Section 8 of the US Constitution) adds the term “invasion,” providing Congress with the power to call “forth the militia to execute laws of the Union, suppress insurrections and repel invasions.” There is precedent for federalizing the National Guard under the Calling Forth Act and even using the armed forces for such purposes.

Despite what you may hear in the media, the Posse Comitatus Act isn’t the sole source governing legal authorizations of domestic military operations. There is more nuance and complexity than that. An overly simplified narrative that the Posse Comitatus Act prevents the armed forces from performing law enforcement functions omits other relevant—and very real—considerations and risks politicizing the military’s role in domestic operations. The reality is that there are constitutional and doctrinal exceptions to the Posse Comitatus Act that are almost entirely left to subjective interpretation and therefore difficult to discredit. Debates about the wisdom or effectiveness of using US military personnel at the border are important, but they should be held within a context that understands and considers the legal components of the issue relative to the public narrative justifying their use.

Dr. Ryan Burke is a professor of military and strategic studies at the US Air Force Academy, codirector of Project 6633 at the Modern War Institute, affiliate professor at the University of Alaska’s Center for Arctic Security and Resilience, and research director for the Homeland Defense Institute at USAFA and USNORTHCOM.

The views expressed are those of the author and do not reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.

Image credit: Staff Sgt. Nathan Akridge, US Army