President Donald Trump’s decision to deploy the military to the US southern border generated heated debate prior to this month’s midterm elections—where immigration policy continued to be a leading agenda item. Media outlets nationwide reported the deployment with a consistent narrative. But almost all of the reporting failed to address the nuance and complexity of the situation. Even some on-air military analysts made superficial statements about what the military can and can’t do at the border.

This isn’t surprising. Save for a small, niche group of scholars and practitioners well versed in the subject, most people in the media and general public (and many in the military, as well) are wholly ignorant to what the armed forces can and can’t do on US soil. With the media abuzz reporting on a military deployment to stop what the president refers to as a migrant caravan “invasion,” this is a timely and relevant discussion; one that requires clarification and facts rather than media reports that are misinformed and misleading at best, and manipulative at worst. And that’s my intent here. I’ll do this by answering two simple questions: 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.

First, though, 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, and Marine Corps. The president is the commander-in-chief of the armed forces 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, DoD can provide immediate support 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 fact that the words “fencing,” “border,” “barrier,” “perimeter,” and even “wall” appear nowhere in the permissible military activities above, the US Army is currently 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 execute law on US soil and is an often-referenced law in the ongoing border support dialogue. The actual wording of the law only mentions the Army and Air Force specifically, but the law’s restrictions do apply to both the Navy and Marine Corps by extension. 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 driving the discussion here is what exactly constitutes “executing law.” According to DoD policy, 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 these legal and policy restrictions, media outlets continue running headlines with phrases like “Military to Stop Migrant Caravan” and “Military to Defend the Southern Border.” Words mean things. In these examples, the words “Stop” and “Defend” evoke images of soldiers and Marines lining the border with rifles ready to engage anyone who dares to cross. So why are these the words chosen in the headlines?

Immigration is a politically divisive topic. Some might argue the headlines are examples of sensationalist media propaganda whereas others might say it is simply ignorance. Regardless of the political agenda, TV, online, and print media sources have—either intentionally or unintentionally—misrepresented the military’s border support efforts. The word support wasn’t emphasized at all until DoD decided to drop the name “Operation Faithful Patriot” in favor of the more generic “border support.” So is the misrepresentation the result of intentional propaganda or unintentional ignorance? I contend it’s ignorance rather than propaganda, which is why it’s so important to set the record straight.

The Debate: Is the Migrant Caravan a Threat?

When a reporter asked President Trump on November 2 whether troops would fire on foreign civilians, his response—“I hope they won’t”—elicited criticism from the media. In an ironic self-contradiction, the media outlets that were reporting the military was going to “defend the border” and “stop the migrants” were now quick with retort claiming that the military couldn’t actually use force at the border. The media changed their own narratives to say troops are there only to offer “logistical support” and that it would be illegal for troops to engage civilians because of the Posse Comitatus Act. Well, that’s not entirely accurate. Remember those express constitutional authorizations and acts of Congress I mentioned? The fact is, if the president or Congress deems the migrant caravan 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. Is the migrant caravan a legitimate threat to our national security? Are the people in the caravan dangerous or do they wish us harm? The answers vary. We know from his comments and tweets the president views the caravan as a threat. And since he does, this can warrant the use of military force to defend against it under the purview of homeland defense. 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 US sovereignty, territory, domestic population, and critical infrastructure against external threats and aggression or other threats, as directed by the President of the US,” and DoD is the lead federal agency in such situations. But the migrant caravan isn’t a threat to the sovereignty of the United States, its territory, or its people, critics might say. Well, the United States government says otherwise, and is legally authorized to do so.

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. But this isn’t the only exception to PCA restrictions.

If we consider the president’s tweets and his answers to reporters’ questions on the subject in recent days, we see and hear the word “invasion” repeatedly. This is undoubtedly a deliberate word choice. In the president’s use of the word, he establishes his assessment of the migrant caravan as a threat to the United States. If the president labels the border support effort a homeland defense operation as necessary to defend against a transnational threat—which he can do—and he labels potential illegal immigration of the masses as a pending invasion—which he has done—it provides a legally defensible reason to use the armed forces and justifies potential invocation of the Insurrection Act.

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. The legal definition of insurrection may be the only obstacle in the way of invoking the Insurrection Act. To qualify as a legal insurrection, the activities must be performed by “citizens or subjects of a country.” Whereas migrants are not citizens, if, say, a group of migrants crosses the US border at a location other than a legal port of entry, they technically become subjects of the United States and have now committed a federal crime, albeit a misdemeanor. While the United States must legally allow such individuals to pursue asylum claims, the migrants may still be detained for breaking US laws; and they may be detained by the armed forces or otherwise acted upon militarily—if, say, they throw rocks at US military personnel—as the situation requires.

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. With that context in mind, the president may use such incendiary terms deliberately to justify his use of the armed forces at the border, or even to justify his eventual calling forth of the National Guard—and it is entirely legal.

So in short, between the president’s ability to declare a homeland defense situation or invoke the Insurrection Act, and Congress’s ability to invoke the Calling Forth Act, it is easy to logically conceive of a border situation in which the National Guard and the armed forces are granted legal permissions to actually “defend the border” and “stop the migrant caravan” by enforcing laws as the president considers necessary.


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. I’ve described how the president and Congress can legally authorize the armed forces to perform law enforcement functions at the border if deemed necessary, provided certain conditions are met. So when the media says that the Posse Comitatus Act prevents the armed forces from performing law enforcement functions, it omits other relevant—and very real—considerations and demonstrates no more than a superficial understanding of domestic military law and policy. The reality is that there are constitutional and doctrinal exceptions to the Posse Comitatus Act that are almost entirely up to subjective interpretation and therefore difficult to discredit. Simply put: if the president considers the migrant caravan a legitimate threat to the sovereignty and security of the United States, and any migrant or group of migrants crosses the border illegally, the president can federalize the National Guard and direct the armed forces to suppress an insurrection or invasion—legally. Debates about the wisdom or effectiveness of such an action are important, but are undermined by media reports that fail to get the question of legality right.

Dr. Ryan Burke is an associate professor of Military & Strategic Studies at the U.S. Air Force Academy and is a former Marine Corps officer. He has written on the subject of military-involved domestic and international support for international journals; the Strategic Studies Institute; and others. He is also the chief editor of Military Strategy, Joint Operations, and Airpower: an Introduction (Georgetown University Press, 2018).

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

Image credit: Staff Sgt. Nathan Akridge, US Army