How Trump Could Deploy the Military for Mass Deportation

This article first appeared at Just Security.

President-elect Donald Trump has promised to carry out the largest deportation operation in American history. While he has provided few specifics about how his administration would accomplish this, Trump has indicated that he will declare a national emergency and that he will invoke both the Insurrection Act and the Alien Enemies Act. A common theme in almost all of his statements is his intent to deploy the military.

The prospect of heavy military involvement in immigration enforcement has generated alarm in many quarters. But the authorities that Trump might summon—and the hurdles he might face in using them—are not widely understood. This article seeks to identify and elucidate them. Starting with the baseline constraint on military participation in law enforcement, the Posse Comitatus Act, the discussion below sets forth the relevant statutory authorities in escalating order of potency and identifies potential constraints on their use.

This article is intended as both a primer and a reference (i.e., readers can skip to the particular authorities about which they may have questions). The authorities covered may be summarized as follows:

• Chapter 15 of Title 10 of the U.S. Code has several provisions allowing federal armed forces (including active-duty forces and federalized National Guard forces) to assist with law enforcement, including through the provision of military facilities and equipment, without directly participating in core law enforcement activities.
• A declaration of national emergency under the National Emergencies Act could provide additional resources for military assistance to law enforcement but would not permit federal armed forces to participate in core law enforcement activities.
• 32 U.S.C. 502(f)(2)(A) allows the president to request (but not require) that governors deploy their states’ National Guard forces to perform certain federal missions, free from the constraints of the Posse Comitatus Act—i.e., Guard forces could participate in core law enforcement activities.
• The Insurrection Act, generally regarded as the primary statutory exception to the Posse Comitatus Act, allows the president to federalize National Guard forces and to deploy them and active-duty armed forces anywhere in the country.
• The Alien Enemies Act does not itself authorize military deployment, but when combined with the Insurrection Act or other authorities, it could permit the militarized deportation of immigrants lawfully in this country under specified conditions.

More on all of these authorities can be found on the Brennan Center’s “Domestic Deployment of the Military” and “Outdated and Dangerous” webpages. 

What Does It Prohibit?

The Posse Comitatus Act embodies a long Anglo-American tradition against military involvement in civilian affairs. It prohibits federal armed forces from acting “as a posse comitatus or otherwise execut[ing] the laws”—i.e., from participating in civilian law enforcement activities, whether criminal or civil—unless “expressly authorized” by Congress or the Constitution.

The Department of Justice has concluded that the Constitution gives the president the “inherent” power to unilaterally deploy the military for law enforcement purposes in some circumstances—for instance, to protect federal personnel or property. A straightforward reading of the Constitution, however, reveals no express authorization for such deployment. Accordingly, any military action that relies solely on these “inherent” powers should be deemed to violate the Posse Comitatus Act, and should be sustained only if it falls within the president’s “conclusive and preclusive” sphere of authority (per Justice Robert Jackson’s concurrence in Youngstown Sheet & Tube Co. v. Sawyer).

What Are the Limitations?

As set out in a recent Brennan Center report, there are multiple exceptions to the Posse Comitatus Act and significant gaps in its coverage:

• The law applies only to federal armed forces. It does not bind the National Guard unless the Guard has been called into federal service. As discussed below, the National Guard may perform federal missions (such as border security) under Title 32 without being federalized.
• Courts have construed the Posse Comitatus Act to prohibit only direct participation in core law enforcement activities, such as arrests, searches, and seizures. A wide range of indirect support to law enforcement—including conducting reconnaissance, sharing intelligence, and providing/maintaining military equipment—remains fair game, even if such support is integral to a law enforcement operation.
• Courts also have held that the Posse Comitatus Act does not apply to actions that have a primarily military purpose, regardless of any incidental benefits to civilian law enforcement authorities. Trump conceivably could try to invoke this doctrine based on his characterization of unlawful migration as an “invasion.” Any such attempt should fail given the absence of an armed attack by a foreign power (see Alien Enemies Act discussion, below).
• There are many statutory exceptions to the Posse Comitatus Act—most importantly, the Insurrection Act, discussed below. 

What Does It Permit?

Chapter 15 of Title 10 of the U.S. Code (which includes the 1981 Military Cooperation With Civilian Law Enforcement Agencies Act and other authorities) authorizes a range of federal military assistance to law enforcement agencies, but does not authorize direct participation in core law enforcement activities. Most notably:

• 10 U.S.C. § 272 allows the Secretary of Defense to make available any military equipment, base facility, or research facility to any federal, state, or local civilian law enforcement official for law enforcement purposes.
• 10 U.S.C. § 273 allows military personnel to provide training to federal, state, and local civilian law enforcement officials in the operation and maintenance of equipment, and to provide expert advice.
• 10 U.S.C. § 274 allows military personnel to maintain equipment on behalf of federal, state, and local civilian law enforcement, and to operate such equipment in specified situations and for specified purposes.
• 10 U.S.C. § 284 authorizes certain types of military support for counterdrug activities or activities to counter transnational organized crime of any federal agency or any state, local, tribal, or foreign law enforcement agency. In the case of domestic agencies, the permissible types of support include:
  • Maintenance and repair of military equipment;
  • Transportation of personnel, supplies, and equipment;
  • Establishment and operation of bases of operations or training facilities;
  • Training of civilian law enforcement personnel;
  • Detection, monitoring, and communication of the movement of air, land, and sea traffic within 25 miles of the geographic boundaries of the United States;
  • Construction of roads and fences and installation of lighting to block drug smuggling corridors across international boundaries;
  • Establishment of command, control, communications, and computer networks for improved integration of law enforcement, active military, and National Guard activities;
  • Provision of linguist, intelligence analysis, and planning services; and
  • Aerial and ground reconnaissance.

These authorities have long been used to enable Department of Defense support of the Department of Homeland Security’s activities at the southern border.

What Are the Limitations/Potential Legal Challenges?

• Title 10 Chapter 15 includes an express statement (10 U.S.C. § 275) that its provisions do not authorize military participation in arrests, searches, seizures, or similar activities. In other words, they are not exceptions to the Posse Comitatus Act.
• Title 10 Chapter 15 also prohibits the provision of support to civilian law enforcement agencies “if the provision of such support will adversely affect the military preparedness of the United States” (10 U.S.C. § 276).
• A significant constraint on these authorities is resource availability and funding. Although the Department of Defense has some flexibility to reprogram the funds allocated by Congress, that flexibility is not infinite—and neither are the resources available to be reprogrammed. In a recent Lawfare article, Professor Chris Mirasola adeptly summarized the authorities for, and limitations on, shifting funds within the Department of Defense.

What Does It Permit?

The National Emergencies Act of 1976 authorizes the president to declare a national emergency. That declaration unlocks enhanced authorities that are set forth in roughly 150 statutory provisions, which the Brennan Center has identified and catalogued. These authorities span almost every area of governance, including military deployment, commerce, transportation, communications, agriculture, and public health.

Trump has indicated that he plans to declare a national emergency, but he has not specified which emergency powers he intends to invoke. (Although his aides reportedly believe an emergency declaration will permit the use of military bases and planes to detain and deport immigrants, military bases and planes would be available without an emergency declaration under Title 10 Chapter 15.) Indeed, it is unclear whether Trump’s advisors have determined—comprehensively and accurately—what authorities might be available. This article will not do that work for them.

Instead, it is instructive to review recent uses of emergency powers for immigration enforcement:

• In 2021, President Joe Biden declared a national emergency to address international drug trafficking. In 2023, pursuant to that declaration, Biden invoked 10 U.S.C. § 12302, which allows him to call up members of the reserve components of the armed forces, including the National Guard. At the time, there were already 2,500 federalized National Guard forces assisting the Department of Homeland Security at the border, and they were soon joined by 1,500 active-duty armed forces. It is unclear how many reservists, if any, were mobilized under the emergency declaration.
• In 2020, under the national emergency declaration for COVID-19, Trump invoked 19 U.S.C. § 1318(b)(1)(C) to close off a large portion of cross-border traffic. That provision allows the Secretary of Homeland Security to temporarily close, relocate, or modify the services provided by any customs office or port of entry during a national emergency.
• In 2019, Trump invoked 10 U.S.C. § 2808 to secure funding for the southern border wall. The law applies during a national emergency “that requires use of the armed forces.” It allows the Secretary of Defense to undertake military construction projects that are necessary to support such use of the armed forces, using unobligated funds appropriated for military construction projects that have been canceled or downgraded. As other observers have noted, Trump might invoke 10 U.S.C. § 2808 again to construct or adapt military bases to serve as immigrant detention facilities.

What Are the Limitations/Potential Legal Challenges?

• None of the emergency powers that Trump might invoke authorizes direct participation in core law enforcement activities. Military personnel could not search, arrest, or detain immigrants under a national emergency declaration.
• The emergency authorities identified above primarily provide additional resources to carry out activities that are otherwise authorized, rather than expanding substantive powers. Even 19 U.S.C. § 1318(b)(1)(C) is not as significant an expansion of power as it might seem, given that another subparagraph of 19 U.S.C. § 1318 permits the same actions without a national emergency declaration “when necessary to respond to a specific threat to human life or national interests.” Generally speaking, when it comes to immigration enforcement, a national emergency declaration is more likely to serve as a resource amplifier than a way to access broad powers that are otherwise unavailable.
• Because the National Emergencies Act includes no definition of “emergency” and no substantive criteria for declaring one, courts will likely be deferential to the president’s judgment that an emergency exists. However, courts can—and do—probe whether the government’s actions are authorized under the specific emergency powers invoked. Most recently, the Supreme Court struck down President Biden’s use of emergency powers (pursuant to the COVID-19 national emergency declaration) to forgive student loan debt.
  • During the first Trump administration, some courts struck down Trump’s use of 10 U.S.C. § 2808 to fund the border wall. These rulings were stayed on appeal and the Supreme Court ultimately vacated them when Biden terminated the emergency declaration, but similar challenges will no doubt occur if Trump invokes emergency powers for immigration enforcement. (Robert Taylor, former Acting General Counsel at the Department of Defense, has explainedsome of the key limitations on the reach of 10 U.S.C. § 2808.)
  • The use of 19 U.S.C. § 1318(b)(1)(C) (or the parallel non-emergency provision) to close off the border would be an overreach under the reasoning set forth in the Supreme Court’s student loan decision—i.e., if Congress intended to authorize complete closure of the border, it would have said so specifically, rather than permitting temporary closures of individual ports of entry.

What Does It Permit?

The National Guard may operate in one of three different statuses:

• In “State Active Duty” status, National Guard forces perform state missions with state funding and operate under the command and control of the state’s governor.
• In Title 10 or “federalized” status, National Guard forces temporarily become a part of the federal armed forces and operate under the command and control of the president until returned to state status.
• In Title 32 or “hybrid” status, National Guard forces perform activities specified by Congress or the federal government and are paid with federal funds, but as a legal matter they remain under the command and control of the state governor.

In Title 32 status, even though Guard forces may be performing a federal mission, they are not subject to the Posse Comitatus Act because they are at least nominally under state command and control. Relevant authorizations under Title 32 include:

• 32 U.S.C. § 112 authorizes funding for the National Guard to carry out drug interdiction and counter-drug activities that serve a state law enforcement purpose, in accordance with a federally approved state plan.
• 32 U.S.C. § 502(f)(2)(A) authorizes the National Guard to perform “operations or missions undertaken … at the request of the President or Secretary of Defense.”
• 32 U.S.C. § 902 authorizes funding for National Guard forces to conduct homeland defense activities approved by the Secretary of Defense. “Homeland defense activities” are defined as activities undertaken for the military protection of U.S. territory, population, or infrastructure from a “threat or aggression” against the United States.

Section 502(f)(A)(2) has played a major role in the increasing militarization of the southern border. Under Presidents George W. Bush, Obama, and Trump, thousands of National Guard forces were deployed under this provision. (Trump also deployed active-duty armed forces; Biden did the same, and he federalized the National Guard forces at the border.) Although National Guard forces in Title 32 status are not subject to the Posse Comitatus Act, they have not directly participated in core law enforcement activities during these deployments. Instead, they have provided support to DHS in the form of surveillance, transportation, provision of equipment, and the erection of barriers.

Trump could seek to expand the use of National Guard forces at the border under Title 32 by involving them directly in the apprehension and detention of migrants. He also could request that governors use their National Guard forces to assist with immigration enforcement in the interior of the country.

What Are the Limitations/Potential Legal Challenges?

• State governors, not the president, decide whether to deploy their National Guard troops under Title 32. Under 32 U.S.C. § 502(f)(2)(A), for instance, the president or Secretary of Defense may “request” deployment of troops for a federal mission, but the governor is free to refuse the request.
  • In 2020, Trump requested that 15 governors send their National Guard troops to Washington, D.C. under 32 U.S.C. § 502(f)(2)(A) to join forces with the D.C. National Guard in quelling protests against the police killing of George Floyd. Eleven governors complied with this request; four declined.
• Under Title 32, a governor may not send the state’s National Guard into another state without the latter state’s consent, even to perform a federal mission. Although the issue has not come before the courts, any such deployment would likely be deemed a violation of state sovereignty under the U.S. Constitution (as Joseph Nunn has explained). As a practical matter, this limits Trump’s use of Title 32 to states that have governors who agree with Trump’s deportation policies.
  • The eleven governors who sent National Guard forces into Washington, D.C. in 2020 did so over the objections of the district’s mayor. That was a distinct circumstance, however, as the District of Columbia does not have the same constitutional status as the states and the D.C. National Guard is under the command and control of the president.
• There is a strong legal argument that 32 U.S.C. § 502(f)(2)(A) is not as broad as its text might suggest—i.e., it cannot be used for literally any operation or mission, but is limited to traditional National Guard functions set forth elsewhere in the law. This argument would be particularly forceful if Trump requested deployment of the National Guard to police protests, but it could also serve as a basis for challenging expanded participation in immigration enforcement.
• National Guard forces operating in Title 32 status are subject to state law, and states generally have laws specifying who may serve as “peace officers” (a status that usually corresponds with the performance of core law enforcement functions). Those laws vary considerably in how restrictive they are. In some states, National Guard units would not be permitted to engage in arrests or similar law enforcement activities unless deputized to do so—a process that is within the state’s discretion and can involve extensive training requirements. 

What Does It Permit?

The Insurrection Act is the most significant exception to the Posse Comitatus Act. Despite its name, its reach is not limited to insurrections. The Act allows the president to deploy active-duty armed forces and to federalize the National Guard for the following purposes:

• to suppress an insurrection in a state, if assistance is requested by the state legislature (or, if the legislature is unable to convene, the state’s governor) (10 U.S.C. § 251);
• to enforce federal law or suppress a rebellion against U.S. authority, whenever the president considers that “unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States” make it “impracticable” to enforce federal law “by the ordinary course of judicial proceedings” (10 U.S.C. § 252); or
• to suppress any “insurrection, domestic violence, unlawful combination, or conspiracy” (10 U.S.C. § 253) that—
  • interferes with the execution of state or federal law in a way that deprives a class of people of constitutional rights, and state authorities are unable or unwilling to enforce those rights; or
  • opposes or obstructs the execution of, or impedes the course of justice under, federal law.

When federalized under the Insurrection Act, National Guard forces (along with active-duty armed forces) may be deployed in any state or territory of the United States. No violation of one state’s sovereignty by another state occurs, as the National Guard is operating as part of the federal armed forces.

Because the statute authorizes the president to call up the “militia,” some have argued that it provides authority for the president to essentially deputize private militias and other groups as federal forces. The term “militia” is defined, under the same title as the Insurrection Act, to include all able-bodied males between the ages of 17 and 45 years of age who are, or who have made a declaration of intention to become, U.S. citizens, as well as female U.S. citizens who are members of the National Guard.

The Insurrection Act has not previously been used for immigration enforcement or border security. Its past uses (approximately 30 of them, as set forth in a guide published by Brennan Center) mostly fall within four categories:

• Suppression of rebellions against state or federal authority (e.g., the Whiskey Rebellion and the Civil War);
• Suppression of labor movements/breaking strikes (e.g., the Pullman Strike of 1894);
• Protection of civil rights, including those of African-Americans during both Reconstruction and the Civil Rights Movement and those of Chinese immigrants in the late 19th century; and
• Quelling civil unrest and, particularly in the late 1960s, “race riots.”

The most recent exercise of the Insurrection Act occurred over 30 years ago, when the Governor of California asked President George W. Bush to deploy troops to quell civil unrest that had erupted after a jury acquitted police officers charged in the beating of Rodney King.

What Are the Limitations /Potential Legal Challenges?

The Brennan Center has called for reform of the Insurrection Act because it lacks the necessary safeguards against abuse. That said, there are a number of ways in which an exercise of the law could be challenged.

• In 1827, the Supreme Court held in Martin v. Mott that the Insurrection Act did not permit judicial review of a president’s decision to deploy troops. In other words, as a general rule, the president alone determines whether the criteria for deployment set forth in the law have been met. However, there are important caveats to this ruling:
  • Language in this and later decisions (most notably, the 1932 case Sterling v. Constantin) suggested that there might be a “bad faith” exception to the general rule of non-reviewability, and that courts may step in if the president has exceeded a “permitted range of honest judgment.”
  • Congress has extensively amended the law since the 1827 decision, and courts could conclude that the revised law—which includes much more detailed criteria for deployment—allows for judicial review in at least some circumstances beyond the “bad faith” scenario.
• If the courts determine that they may review whether deployment criteria are met, they should adopt long-standing Office of Legal Counsel interpretations that construe those criteria narrowly, in keeping with constitutional principles and tradition. According to these interpretations, invocation of the Insurrection Act must be a “last resort,” occurring only when a state requests assistance to put down an insurrection; when necessary to enforce a federal court order; or when “state and local law enforcement have completely broken down.”
• Even if courts determine that they may not review whether the legal criteria for deployment have been met, Supreme Court precedent strongly suggests that they may review whether the military’s actions pursuant to deployment are lawful. As the Court affirmed in Sterling: “What are the allowable limits of military discretion, and whether or not they have been overstepped in a particular case, are judicial questions.”
  • The Insurrection Act does not (and could not) authorize the violation of constitutional rights. For instance, if soldiers deployed under the Insurrection Act searched homes without a warrant or an applicable exception to the Fourth Amendment’s warrant requirement, courts could intervene. Prolonged military detention under the Insurrection Act would similarly be subject to constitutional challenge.
  • If there is a conflict between the Insurrection Act and other federal laws, the later-enacted and more specific law should prevail (in accordance with standard rules of statutory interpretation). Given the age and sweeping scope of the Insurrection Act, most federal laws will be later-enacted and more specific. The 1948 federal law restricting the presence of military personnel at polling stations, for instance, should be read as a limitation on deployment under the Insurrection Act.
  • A 1975 Department of Justice memorandum suggested that actions taken under the Insurrection Act must be “reasonable under the circumstances.” Although the Insurrection Act is generally understood as an exception to the Posse Comitatus Act, the same Department of Justice memorandum questioned whether the law authorizes arrests, as opposed to temporary detentions of civilians until such time as they can be turned over to civilian authorities.
• Professor William Banks has made the case that using federal armed forces to quell domestic violence without a state’s request for assistance would violate Article IV of the Constitution, which requires the United States to protect states against “domestic Violence . . . on Application of the [state] Legislature, or the [state] Executive (when the Legislature cannot be convened).”

What Does It Permit?

The president can invoke the Alien Enemies Act if Congress has declared war or if the president proclaims an “invasion” or “predatory incursion”—or a threat of such invasion or incursion—by a foreign nation or government. The Act authorizes the president to summarily detain and/or deport any non-U.S. citizens who were born in the enemy nation(s), including those lawfully present in the United States, regardless of whether there is any reason to suspect that they are dangerous.

The Alien Enemies Act was last used in World War II to implement the internment of tens of thousands of non-U.S. citizens of Japanese, German, and Italian descent. (U.S. citizens of Japanese descent were interned under a separate authority.) Congress and the Executive Branch have since issued apologies for much of this shameful episode in our nation’s history, acknowledging that internment under the Alien Enemies Act discriminated against people based on their ancestry.

Trump has threatened to use the Alien Enemies Act to deport Latino immigrants who are undocumented or involved in criminal drug activity. Ostensibly, he intends to claim that migration from Mexico and other countries south of the border constitutes an “invasion” or “predatory incursion” perpetrated by drug cartels operating as de facto governments in those regions.

Notably, the Alien Enemies Act does not itself authorize military participation in law enforcement. If the military were deployed under other authorities (such as the Insurrection Act), however, Trump could attempt to use the Alien Enemies Act to expand those uses of the military to include the summary detention and deportation of certain immigrants who are lawfully in this country—something none of the other authorities discussed above could do.

What Are the Limitations/Potential Legal Challenges?

• As detailed in a recent Brennan Center report, the design and history of the Alien Enemies Act leave no doubt that it is a wartime authority only. It is intended to address armed attacks by foreign powers, not people fleeing persecution or seeking economic opportunity. (Legal historians such as Professor Ilya Somin and Professor Frank Bowman have similarly explained that the term “invasion” in the Constitution does not apply to migration.) If Trump invokes the law for immigration enforcement purposes, courts can and should deem this to be a staggering abuse.
• Courts similarly should reject any effort to characterize drug cartels as de facto governments—a move that would have significant unintended consequences for U.S. diplomatic relations and policy in the region and beyond.
• The Brennan Center’s report also argues that the Alien Enemies Act, which targets individuals based solely on their ancestry and dispenses with the procedural protections of regular immigration law, is inconsistent with modern understandings of the Constitution’s equal protection and due process protections.