President Trump has turned a 60-foot-wide strip of federal land that spans three states on the southern border into a “military installation” to “address the emergency” he previously declared over unlawful immigration and drug trafficking. Trump’s memo authorizing this action seems designed to sidestep the Posse Comitatus Act, which normally bars federal armed forces from conducting domestic law enforcement. The apparent plan is to let the military act as a de facto border police force, with soldiers apprehending, searching, and detaining people who cross the border unlawfully.
This move could have alarming implications for democratic freedoms. Moreover, it continues a pattern of the president stretching his emergency powers past their limits to usurp the role of Congress and bypass legal rights. He has misused a law meant to address economic emergencies to set tariffs on every country in the world. He declared a fake “energy emergency” to promote fossil fuel production. And he dusted off a centuries-old wartime authority to deport Venezuelan immigrants, without due process, to a Salvadoran prison notorious for human rights violations.
As presidential overreaches pile up, they underscore the urgent need for Congress and the courts to reassert their roles as checks on executive authority.
The Posse Comitatus Act and the Military Purpose Doctrine
Last week, the military announced that soldiers deployed on the New Mexico–Mexico border will have “enhanced authorities” because they are on land that has now been designated part of Fort Huachuca, Arizona — a military installation located more than 100 miles away. The new authorities include the power to “temporarily detain trespassers” on the “military installation” and “conduct cursory searches of trespassers . . . to ensure the safety of U.S. service members and Department of Defense (DoD) property.”
Searching and apprehending migrants would ordinarily run afoul of the Posse Comitatus Act, which prohibits federal armed forces from directly participating in civilian law enforcement activities unless doing so is expressly authorized by Congress or the Constitution. The law stems from an Anglo-American tradition, centuries older than the Constitution, of restraining military interference in civilian affairs. It serves as a critical check on presidential power and a vital safeguard for both personal liberty and democracy.
Nonetheless, several exceptions exist. The most significant is the Insurrection Act — a law that Trump floated using to address unlawful migration (although for now, his secretaries of defense and homeland security are reportedly recommending against such a move). In authorizing soldiers to conduct apprehensions and detentions on lands that have been newly designated as a “military installation,” the president is relying on a lesser-known loophole in the Posse Comitatus Act known as the “military purpose doctrine.”
The doctrine, conceived by the executive branch and endorsed by the courts, holds that an action taken primarily to further a military purpose does not violate the Posse Comitatus Act even if it provides an incidental benefit to civilian law enforcement. A textbook example is the circumstance in which a person has driven drunk onto a military base. Soldiers may legally detain the intruder until civilian law enforcement arrives to take them into custody. This does not violate the Posse Comitatus Act because the primary purpose of the military’s activity is not to enforce the laws against driving while impaired, but to maintain order on the base and protect military assets and personnel.
Having turned much of the southern border into a “military installation,” the administration now takes the position that anyone crossing the border without authorization in those areas is not just violating immigration law but also trespassing on a military installation. Federal troops thus have a legitimate military reason, the argument goes, to apprehend, search, and detain migrants without violating the Posse Comitatus Act and without the president needing to invoke the Insurrection Act at all.
Evaluating the Legality of the Order
Using the military purpose doctrine to justify direct military involvement in immigration enforcement is a transparent ruse to evade the Posse Comitatus Act without congressional authorization. The doctrine is meant to apply only in cases where any law enforcement benefit is purely incidental. Here, the situation is the opposite.
The nominal justification for apprehending and detaining migrants who cross the border is protecting the installation. But the installation itself was created to apprehend and detain migrants, as well as to secure their removal. In the memo, this is described as “sealing the border” and “repelling the invasion” at the border. No matter how the Trump administration frames those activities, however, they are civilian law enforcement functions. He cannot turn them into military operations by misusing the language of war. These civilian law enforcement activities are not “incidental” — they are the reason for creating the installation. And apprehending migrants who “trespass” on the installation is the primary way in which this law enforcement mission will be furthered.
This use of the military is fundamentally different from the border deployments that have occurred in recent presidential administrations, from George W. Bush to Joe Biden. The military’s role until now has been limited to logistical support, such as assisting border agents with surveillance, infrastructure construction, and transportation. Providing such support does not constitute direct participation in law enforcement, so it does not violate the Posse Comitatus Act. Having soldiers perform core law enforcement duties like apprehending or detaining people, however, steps over the legal boundary.
The move also skirts a separate statute requiring congressional approval of any Pentagon takeover of more than 5,000 acres of federal lands except “in time of war or national emergency.” Here, in order to transfer control of land from the Interior Department, Trump is relying on a declaration of national emergency he issued on January 20 to address the “invasion” at the southern border, in which he asserted that “our southern border is overrun.” But on March 2, Trump triumphantly posted on social media that “the Invasion of our Country is OVER,” adding that in the preceding month, “very few people came.” His administration continues to tout the fact that unlawful border crossings are now at their lowest level in decades. U.S. Customs and Border Protection data from March confirms a 95 percent decline in monthly unauthorized crossings.
Leaving aside the question of whether an emergency, properly defined, existed on January 20 (it didn’t), the Trump administration has made a powerful case that there is no emergency now. Trump should be revoking the emergency declaration, not relying on it to transfer federal lands to the Department of Defense.
Why It Matters
Aside from legal concerns, there are practical reasons why the U.S. armed forces shouldn’t be enforcing immigration law. Federal troops are trained to fight and destroy an enemy; they’re generally not trained for domestic law enforcement. Asking them to do law enforcement’s job creates risks to migrants, U.S. citizens who may inadvertently trespass on federal lands at the border, and the soldiers themselves. And it pulls our armed forces away from their core mission of protecting the United States from foreign adversaries at a time when the military is already stretched thin.
Using the military for border enforcement is also a slippery slope. If soldiers are allowed to take on domestic policing roles at the border, it may become easier to justify uses of the military in the U.S. interior in the future. Our nation’s founders warned against the dangers of an army turned inward, which can all too easily be turned into an instrument of tyranny.
Trump’s misuse of emergency powers similarly has larger implications. Emergency declarations unlock enhanced powers contained in 150 different provisions of law. Many of these are far more potent than the ability to transfer federal lands to the Department of Defense. They include the authority to take over or shut down communications facilities, freeze Americans’ assets, and control domestic transportation. If emergency powers can be invoked for border security at a time when unlawful border crossings have reached a historic low, there is little to prevent a president from declaring fake emergencies to invoke these alarming powers.
The Role of the Courts and Congress
Unfortunately, the president’s abuses could be difficult to check. The Posse Comitatus Act is a criminal statute, and those who violate it may be prosecuted. But it’s unclear whether violations may serve as a basis for migrants to challenge their detention. As for Trump’s misuse of emergency powers, courts generally are reluctant to probe a president’s decision that an emergency exists (although in this case, the administration’s own statements might be sufficient to overcome the presumption of deference). And as the law currently stands, it is very difficult for Congress to terminate a national emergency declaration or undo actions that presidents take using their emergency powers.
These challenges highlight the urgent need for Congress to establish meaningful checks on the use of emergency powers and domestic deployment authorities. Last year, legislation that would have made it much easier for Congress to terminate emergency declarations passed out of committees in the House and Senate with near-unanimous support from both Democrats and Republicans. Similar legislation was introduced in January by Republican Rep. Andy Biggs of Arizona. The Brennan Center has also proposed several key changes to the Posse Comitatus Act that would close the loopholes that threaten to swallow the law.
It may not be possible to pass these reforms soon, but the fight against executive overreach is not just a short-term one. Understanding the ways in which Trump’s actions threaten the rule of law today can help build support for enacting reforms in the future.
