The nation is bracing itself for what President-Elect Donald Trump has promised will be the largest deportation effort in American history. Trump has vowed to use the military to assist with deportations, relying on emergency and wartime powers such as the Insurrection Act, the National Emergencies Act, and the Alien Enemies Act. In addition to worrying about the impact on immigrant families, wider communities, and the economy, many Americans are wondering—is this legal?
The deportation of undocumented individuals who are ineligible for asylum or other legal protection is, of course, well within the government’s authority under current immigration law. (As a policy matter, President Joe Biden has chosen to focus on those who have committed serious crimes—a policy that Trump is set to undo, presumably to facilitate broader deportation efforts.) But deploying the military raises an entirely different set of legal questions. Even under the potent authorities Trump has cited, the actions he proposes to take would, at a minimum, be an abuse of power, and they might well be illegal to boot.
Some degree of military involvement in immigration enforcement is already permitted—and has occurred under multiple administrations—without recourse to emergency powers. This may be surprising to many Americans. Anglo-American law has a long tradition of military noninterference in civilian affairs, for the simple reason that an army turned inward can quickly become an instrument of tyranny. In the United States, this tradition finds expression in an 1878 statute, the Posse Comitatus Act, that prohibits federal armed forces from participating in law-enforcement activities unless expressly authorized by law. Although not every American is familiar with the act, the principle it enshrines is deeply embedded in the public consciousness.
Less well known is the fact that the Posse Comitatus Act is riddled with exceptions and loopholes. For one thing, courts have construed the law to bar only direct participation in core law-enforcement activities, such as arrests or seizures. Federal forces may still provide indirect support to law-enforcement agencies in a number of ways, including conducting reconnaissance, sharing intelligence, and furnishing and operating equipment. In the 1980s, Congress passed several laws authorizing active-duty armed forces to provide these types of assistance.
In addition, the act applies only to federal armed forces. It does not apply to the National Guard—military units within the states that usually operate under state authority—unless the president has called Guard forces into federal service, at which point they become part of the federal military. Congress has passed a law authorizing Guard forces to perform federal missions at the request of the president or secretary of defense even when they haven’t been called into federal service. (Governors have the right to refuse such missions.) The Posse Comitatus Act does not apply to these operations, because the Guard forces remain, at least nominally, under state command and control.
These gaps in the act’s coverage have enabled military involvement in the enforcement of immigration and customs laws at the U.S.-Mexico border for decades, beginning in the 1980s and ramping up after 9/11. Presidents George W. Bush, Barack Obama, and Trump sent thousands of National Guard forces to the border, where they provided support to the Department of Homeland Security in the form of surveillance, transportation, equipment, and the erection of barriers. Trump also deployed active-duty armed forces, as did President Biden. In the summer of 2023, 2,500 National Guard forces and 1,500 active-duty armed forces were stationed at the border.
The seemingly permanent militarization of the U.S.-Mexico border may not violate the Posse Comitatus Act, but it has led to a variety of harms. When thousands of soldiers are routinely arrayed at the border, Americans receive the message that migrants are a threat to national security and public safety—a baseless notion that underlies and fuels support for Trump’s anti-immigration platform. Prolonged deployments at the border are also bad for the military, as they undermine service members’ morale and divert resources and personnel from core military functions.
Trump now reportedly seeks to double down on the militarization of immigration enforcement by invoking a trio of emergency authorities, beginning with the Insurrection Act of 1807—the primary statutory exception to the Posse Comitatus Act. The Insurrection Act gives the president broad powers to deploy federal armed forces (including the federalized National Guard) to quell civil unrest or enforce the law. The criteria for deployment are written in vague, archaic terms that provide few clear constraints. To make matters worse, the Supreme Court held in 1827 that the president is the sole judge of whether the criteria for deployment have been met. In other words, courts generally cannot review a president’s decision to invoke the law.
Although a top aide has said that Trump will invoke the Insurrection Act, the Trump team has provided scant detail on how he plans to use federal forces once deployed. Given that nonemergency authorities already authorize substantial military support to civilian law enforcement, it’s conceivable that Trump’s purpose in invoking the Insurrection Act is purely symbolic—a performative act of “shock and awe.” (The very name of the law suggests that immigrants are attacking from within and must be defeated through force.) At least in theory, though, the law could allow federal forces to perform core law-enforcement functions, such as apprehending and detaining immigrants, in any state in the country and against any governor’s wishes.
Such a use of the Insurrection Act would go beyond a mere expansion of existing military activities. Soldiers rolling into American towns in armored vehicles, knocking on doors, and carting people off to military detention facilities would create risks and harms that current border operations do not. For one thing, direct interactions between military personnel and civilians in fraught circumstances carry a significant potential for violence. After all, soldiers are trained to fight; few receive training in how to peaceably enforce civilian laws while respecting civil liberties. Furthermore, the visible presence of soldiers deployed in the streets would be both alarming and chilling for many Americans. Some would undoubtedly feel less comfortable engaging in protests against Trump’s policies or other basic acts of personal expression.
Heavy involvement of the military in immigration enforcement would also require a massive infusion of resources, both financial and human. That’s where Trump’s plan to declare a national emergency might come in. Under the National Emergencies Act, presidential declarations of national emergency unlock enhanced powers contained in 150 provisions of law spanning almost every area of governance, including military deployment, commerce, transportation, communications, agriculture, and public health. These provisions can supply both additional authority and additional resources for presidential action in a crisis.
Trump has used these powers before. In 2019, Trump declared that unlawful migration at the southern border constituted a national emergency. He invoked an emergency power that frees up funding for “military construction” projects, which he used to secure funds Congress had refused to allocate for the border wall. He might well reprise this effort, and he could attempt to use the same provision to fund the construction of military bases that would serve as immigrant-detention facilities. He could also use emergency powers to call up reservists, amplifying the manpower available to detain and deport immigrants. Indeed, Biden did exactly that in 2023 to supplement forces at the southern border.
Finally, Trump has pledged to invoke the Alien Enemies Act—the last remaining vestige of the notorious 1798 Alien and Sedition Acts. A president may invoke this law when Congress has declared war or when the president proclaims an “invasion” by a foreign government. It allows the president to detain and deport immigrants, including green-card holders and others lawfully in the country, who are not U.S. citizens and who were born in the enemy nation. Immigrants targeted under the act are not entitled to the hearings and other procedural protections afforded by immigration law.
The act was last used in World War II to implement the internment of more than 31,000 noncitizens of Japanese, German, and Italian descent. (U.S. citizens of Japanese descent were detained under a separate authority.) Congress and the U.S. government have since apologized for much of this shameful episode in our nation’s history.
According to reporting in Rolling Stone, Trump may claim that migration from Mexico and other countries south of the border constitutes an “invasion” perpetrated by drug cartels that are operating as de facto governments in those regions. The Alien Enemies Act does not itself authorize military deployment, but it could be combined with the Insurrection Act and other authorities to significantly expand the military’s remit. Most notably, if Trump were successful in invoking these laws, they could allow troops to detain and deport not just undocumented individuals but people who are lawfully present in the United States.
There is no question that the authorities Trump has cited grant the president sweeping powers. The Brennan Center, where I work, has called attention to the dangers posed by each of them. My colleagues and I have urged Congress to reform the laws in order to incorporate safeguards against presidential overreach (or, in the case of the Alien Enemies Act, to repeal it).
But there is also no question that Trump’s proposed actions, as he and his allies have framed them, would be a staggering abuse of these authorities—and quite possibly illegal. Despite the permissive language of the Insurrection Act, it was clearly intended for crises that could not be solved by civilian government actors. That is why it has been invoked only 30 times in the nation’s history and has lain dormant for the past 33 years. In keeping with tradition and constitutional principles, the Justice Department has interpreted the law narrowly, asserting that it should be used only as a “last resort”—specifically, when state and local authorities request military assistance, are obstructing federal law, or have “completely broken down.”
There are many ways to address unlawful immigration short of deploying federal troops. Last spring, for instance, the Senate voted twice on a bipartisan bill that would have dramatically tightened border security. Republicans blocked the measure—reportedly at Trump’s behest, so that he could continue to make the porous border a central focus of his campaign. Having actively obstructed an effort to ramp up civilian enforcement of immigration laws, Trump can hardly argue that military deployment is a “last resort.”
His cynical behavior could open the door to a legal challenge. Although the Supreme Court has generally barred judicial review of Insurrection Act invocations, it has suggested on various occasions that there might be an exception for deployments undertaken in bad faith. That’s because all of the president’s actions, even those committed to his discretion under Article II of the Constitution, must be consistent with the express constitutional obligation to faithfully execute the law.
In addition, the Supreme Court has distinguished between a president’s decision to invoke the Insurrection Act (which is usually not subject to judicial review) and any actions taken by the military after deployment (which are squarely within the courts’ purview). Soldiers deployed under the act must comply with the Constitution and other applicable federal law. If people’s legal rights were violated under a Trump-ordered deployment—for instance, if military detention conditions failed to meet basic human needs—courts would be able to intervene.
Just as invoking the Insurrection Act would be inconsistent with the law’s intent, declaring a national emergency would be a misuse of emergency powers. To be sure, America’s broken immigration system has led to unprecedented numbers of unlawful border crossings. Emergency powers, however, are designed to address sudden, unexpected crises that can’t be handled by Congress through ordinary legislation. There is nothing sudden or unexpected about the problems at the southern border, and Congress can—and should—address those problems through reform of the immigration system.
As a legal matter, courts will be reluctant to second-guess Trump’s decision to declare an emergency. But they will be less deferential in reviewing whether his administration’s actions are authorized under the specific powers he invokes. Although Trump has not identified which powers he plans to use, none of the 150 provisions available during a national emergency is designed to facilitate deportation. Trump will likely be stretching some of these laws beyond their permissible limits. (During his first administration, some courts struck down his use of the military-construction authority to build the border wall.) Courts will also review whether the actions Trump takes pursuant to a national-emergency declaration comport with other federal laws and constitutional rights.
Perhaps the most glaring abuse would be invoking the Alien Enemies Act. The history and design of the law make clear that it is a wartime authority only. It was intended to address armed attacks by foreign nations, not people fleeing political persecution, drug- and gang-related violence, or economic hardship. Even if a significant portion of migrants were criminals—a myth contradicted by all available evidence—that would not render their border crossing an act of war.
Moreover, whether in wartime or peacetime, the Alien Enemies Act suffers from grave constitutional flaws. It permits the targeting of individuals based solely on their ancestry, rather than their conduct, and it allows those individuals to be detained and deported without a hearing. As a recent Brennan Center report argues, these powers are fundamentally inconsistent with modern understandings of constitutional equal-rights and due-process protections.
Whether the Supreme Court would uphold the actions Trump has threatened is impossible to say with any certainty. In recent years, the Supreme Court has occasionally taken positions previously thought inconceivable, and overturned numerous long-standing precedents. But regardless of how the Supreme Court may rule, these actions should rightly be understood as an abuse of power, an abuse of the public trust, and an abuse of the law. And as soon as there is an opportunity, Congress must reform the emergency authorities in question so that no president can ever commit such abuses in the future.
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