The Real Legal Limits on Domestic Military Deployments

Editor's note: This article originally appeared in The UnPopulist.

Few principles of American government are more foundational than the idea that the U.S. military should not be used against Americans, except in the most dire of circumstances. But even fewer foundational principles are so loosely grounded in the law.

The Constitution itself does not bar the domestic use of the military; instead, it vests authority over such deployments in Congress. Congress has in turn limited some such uses while authorizing many others, often through open-ended language that gives the president substantial discretion in determining when and for how long he may use soldiers domestically.

The fact that past presidents have done so relatively rarely is more a product of longstanding political norms than hard legal limits. But with former President Donald Trump—who repeatedly sought to make domestic use of the military in the past and suggested in the final days of his campaign it should be used against “the enemy from within”—these norms may not hold.

This does not mean, however, that domestic military deployments are the shortcut to unmitigated presidential power that Trump—or other power-hungry politicians—think they are. There is no exception to the Constitution for the military when deployed domestically, and soldiers operating within the United States are largely subject to the same (if not greater) legal limits as other federal personnel. In the absence of reforms to the statutes authorizing the domestic deployment of the military, understanding and preparing to enforce these underappreciated legal limits may well be the best means of mitigating abuse.

When Can the President Deploy the U.S. Military Domestically?

Nothing in the Constitution bars the president from making domestic use of the military. If anything, in giving Congress the authority to “call forth” the militias to “execute the Laws of the Union” and “suppress Insurrections”—and making the federal government responsible for protecting each state “against domestic Violence” upon application of its legislature (or executive where the legislature cannot meet)—the Constitution anticipates such deployments as a possibility. The Framers are widely understood to have intended for these steps to be used “only as a last resort” where other means of maintaining domestic stability and the rule of law had failed. But their main approach to limiting the abuse of such authority was not to install hard constitutional restrictions, but to instead vest that authority in Congress, the most democratic branch of government.

Congress has in turn put some legal limits on domestic uses of the military while authorizing others. The most significant limitation is the Posse Comitatus Act, which makes it a crime to use federal military personnel “as a posse comitatus”—a common law term that describes a group of civilians mobilized by officials to enforce the law and keep the peace—”or otherwise to execute the laws” where not expressly authorized by the Constitution or Congress. But this does not operate as the sort of categorical restriction that many assume it to be. Originally enacted in 1878 to limit the federal use of the military to enforce the rights of freed Black Americans in the post-Civil War American South, no one has ever actually been charged under it. And perhaps more importantly, Congress has not been shy about installing exceptions.

The most long-standing exception is the Insurrection Act, the original form of which dates back to the second Congress and thus predates the Posse Comitatus Act itself. Today, the Insurrection Act authorizes the president to use the armed forces wherever:

1. There is an “insurrection in any State against its government” and its legislature (or governor if the legislature cannot be convened) requests assistance;
2. “[U]nlawful obstructions, combinations, or assemblages” or outright rebellion, make it “impracticable” to enforce federal law through ordinary proceedings; or 
3. “[I]nsurrection, domestic violence, unlawful combination, or conspiracy” either hinders the execution of federal or state law in a way that deprives a group of people of federal rights (and that state authorities fail to adequately address), “opposes or obstructs” the execution of federal laws, or “impedes the course of justice” under those laws.

In each case, the president must first issue a public proclamation making the (somewhat antiquated) demand that “the insurgents … disperse and retire peaceably to their abodes” within a fixed timeframe, giving them a chance to back off before facing U.S. military force.

Congress has also expressly authorized the use of certain military personnel to support counter-narcotics and select other law enforcement operations, in roles that—as interpreted by the Justice Department—do not involve “direct, physical confrontation between military personnel and civilians.” Other statutes allow the president or other senior executive branch officials to request military assistance in enforcing criminal provisions prohibiting violence against various foreign and senior U.S. officials. Another provision authorizes the same for criminal provisions prohibiting the unauthorized transfer of nuclear material.

Nor does the Posse Comitatus Act apply to all U.S. armed forces equally. The Coast Guard is excluded from the Act and thus not barred from performing law enforcement functions. Neither are members of the various state National Guard forces when acting under the command of their state governors. The latter, however, become subject to the Act when called into federal duty, which various federal laws (including the Insurrection Act) permit in response to certain emergencies.

Other provisions of law permit governors to volunteer National Guard personnel under their command “to perform training or other duty” as directed (and often funded) by the federal government. This sort of hybrid arrangement has allowed National Guard personnel to be used for purposes ranging from election security to pandemic emergency response at the direction of the federal government, even as they remain under the technical command of their governors. This is how the Trump administration deployed National Guard troops—volunteered by Republican governors, in response to a request from the president—to Washington, D.C., in 2020 to suppress Black Lives Matter protesters, though some doubt whether this use was actually consistent with the statute. As such hybrid personnel are technically operating under state authority, they are also not subject to posse comitatus limitations.

The Constitution itself may similarly exempt certain military personnel from posse comitatus restrictions, either by authorizing the domestic use of the military for non-law enforcement purposes or by giving the president some exclusive constitutional authority to do so. Defense Department regulations identify several such circumstances, including to “quell large-scale, unexpected civil disturbances” where prior presidential authorization is impossible and in cases of “homeland defense.” Few, however, have ever been directly relied upon.

For its part, the Supreme Court has made clear that “the authority to decide whether [an] exigency has arisen” under the Insurrection Act “belongs exclusively to the President,” whose “decision is conclusive upon all other persons”—a deferential approach federal courts seem likely to emulate in relation to other similar authorities. Combined with the broad wording of these provisions—as well as their lack of any time or other limitations—such deference may well allow the president to exercise substantial discretion over when to make domestic use of the military, even if historical context suggests that a narrower interpretation of these authorities would be more appropriate.

The fact that past presidents have not done so more often reflects practical and political, rather than legal, constraints. Soldiers are expensive and trained for missions that do not always translate into domestic applications, making them a more imperfect tool than many acknowledge. By taxing finite resources, domestic deployments can also diminish the military’s readiness for its more foundational national defense mission. And both public and elite opinion—including among senior U.S. military leaders—have generally had an aversion to the domestic use of the military, making it a politically risky move outside of truly exigent circumstances.

Yet these considerations are largely subjective and context-dependent. A president determined to make domestic use of the military, surrounded by loyalists and confident in the unconditional backing of their supporters, might well decide to make more frequent use of the Insurrection Act and related authorities. And if they did, it is not clear that the law would stop them.

Legal Limits on How Domestic Deployments Can Be Used

The mere fact that the president can deploy U.S. soldiers domestically does not, however, mean that he can use them in the same way he might on a foreign battlefield. Domestic uses of the military are subject to far more legal restraints than those overseas, not least because domestic deployments necessarily implicate countless more U.S. persons with constitutional rights. These legal limits have rarely been probed by the courts, in part because past presidents have only pursued domestic deployments sparingly. But together they suggest that increased domestic use of the military is unlikely to be the fast-track to unfettered power that a president might wish it to be.

Domestic uses of the military are still subject to the same constitutional limitations as other forms of federal action, ranging from the need to respect First Amendment speech rights to the Third Amendment prohibition on quartering soldiers in private homes to Fourth Amendment warrant requirements for searches and seizures. While federal courts may be more deferential to the executive branch when addressing purported emergencies—and some constitutional rights have limited exceptions that might apply in certain exigent circumstances—the mere invocation of the Insurrection Act or some similar statutory authority cannot circumvent these constitutional protections.

Both federal and state laws can also impose criminal and civil penalties on domestically deployed military personnel, just as they can for other federal personnel. The soldiers involved in the two contemporary cases in which domestically deployed U.S. soldiers have killed Americans—the 1970 Kent State shootings and the 1997 killing of Esequiel Hernandez on the Mexico-U.S. border—were criminally investigated for their actions and those involved in the former were charged and tried but ultimately acquitted. Following the Kent State massacre, the U.S. Supreme Court unanimously confirmed that both the soldiers involved and other responsible officials could be held civilly liable for their actions. Those harmed in both cases brought lawsuits that eventually ended in substantial settlements.

There are also federal laws that impose certain limits on soldiers that go above and beyond those applicable to other federal personnel. For example, federal law makes it a crime to deploy uniformed military personnel to polling stations during elections. There is no exception to this restriction for following military orders, except where “necessary to repel armed enemies of the United States.” Anyone who violates this provision outside of these circumstances, even pursuant to a presidential order, could find themselves criminally liable.

The greatest limitation on the domestic deployment of military personnel, however, may be the broader limits that the Constitution places on federal authority itself. Outside of conditions of invasion or rebellion, the Insurrection Act and similar authorities generally only authorize the deployment of military forces to enforce federal law. And the substantive scope of federal law is limited, as the Constitution quite deliberately entrusts many key public responsibilities to the states. Efforts to use troops mobilized under the Insurrection Act or another related authority to act in excess of federal law—and particularly in areas of authority entrusted to the states—would thus be inconsistent with the authorizing statutes, as well as the Constitution itself.

These legal constraints by no means suggest that domestic deployments cannot be abused. Nor do they counteract the psychological significance of even legal domestic military deployments, which may intimidate Americans and chill legitimate exercise of rights. But they do set limits on some of the most abusive ways presidents may try to use domestically deployed soldiers.

As an example, take Trump’s recent suggestion that the military should be used to guard against “[r]adical left lunatics” and other “enem[ies] from within” on Election Day—a comment that has been widely criticized as a veiled suggestion that, if reelected, he may well use domestic military deployments to impact elections in 2026 or 2028. Deploying armed and uniformed personnel at polling locations would put those involved in violation of federal law. And any effort to interfere with those elements of election management vested in state authorities by the Constitution—for example, by seizing ballot boxes—would almost certainly be in excess of federal law and thus ripe for a potential injunction.

Of course, these legal restrictions will require litigation and judicial enforcement to be effective. But federal courts can move quickly where needed, including by issuing preliminary injunctive relief that can serve to minimize the harms of presidential abuse pending a final resolution of the underlying legal questions. And as post-2020 election challenges served to demonstrate, the federal courts—and even the current Supreme Court—remain tough terrain for those seeking to curb election results in a way that is clearly contrary to federal law.

Hedging Against the Risk of Abuse

Reports suggest that, during his prior term in office, President-elect Trump repeatedly expressed an interest in domestically deploying the military to suppress protests, patrol the border, and advance other administration interests. While military leaders and other advisors often dissuaded him from taking these steps, this may not be the case in a second Trump presidency. Policymakers and private citizens alike need to prepare for the possibility that domestic deployments will become a more common occurrence—and that they may be used in unprecedented or even abusive ways.

Reforming the federal statutes that authorize domestic deployments such as the Insurrection Act would undoubtedly be the most effective way to curb potential abuse before it happens. Advocates and experts from across the political spectrum have put forward a diverse array of reform proposals, most of which would put tighter limits on the circumstances and periods of time in which the president could invoke that act or otherwise evade posse comitatus restrictions without the approval of Congress. The challenge, of course, is not only crafting a legislative solution but securing enough support in both chambers of Congress to pass it with a large enough margin to overcome a near certain presidential veto.

There are also steps state legislatures could take. While they cannot restrict what a president might do with federalized National Guard troops, state legislatures likely can limit how their governor uses National Guard soldiers on state or hybrid duty, including by restricting the federally funded and authorized activities for which they may be volunteered. Individual states and the District of Columbia could also build upon the existing interstate compact used to coordinate the use of National Guard forces to better reinforce each state’s rights, including by committing not to volunteer National Guard forces for missions within a state not authorized or requested by that state’s relevant authorities, except perhaps for specific legitimate purposes (like enforcing otherwise neglected constitutional rights). Again, however, present political conditions may make it unlikely that enough states will participate in such efforts to meaningfully curb the risk of abuse.

Absent such reforms, those concerned about potential domestic deployments and their abuse will need to focus on the legal limits that already exist that include not only when the military can be deployed domestically and how it may be used. Public advocacy organizations need to make clear and effective plans to pursue a quick and credible legal challenge against any domestic uses of the military that push the limits of the president’s legal authority. This will require both a process for identifying credible plaintiffs with standing—most likely from among opposed state authorities, affected individuals, or even deployed soldiers—and legal counsel prepared to seek emergency injunctions in appropriate state and federal courts as soon as potentially unlawful action appears imminent. Threatening a swift, high-profile, and well-coordinated legal challenge may even help deter the executive branch from attempting abusive domestic deployments in the first place.

Steps should also be taken to enhance public awareness of the legal limits on what a president may do with domestically deployed military forces. Advisors to the president and other federal officials may be less likely to propose domestic deployments as a policy option if they understand the legal limits on their use. State leaders may be similarly less willing to volunteer their hybrid status National Guard members for federally led missions that push legal limits, especially given the potential for civil and criminal liability. And soldiers aware of these legal limits—many of which are already reflected in current military directives and regulations—are more likely to interpret their orders in a manner consistent with the law where possible, and to elevate legal concerns up their chain of command where not.

Indeed, understanding these legal limitations could be particularly important for members of the military who may themselves be domestically deployed. The military justice system imposes serious penalties on members of the armed forces who disregard the chain of command, which runs through the military and Defense Department up to the president. But this obligation only extends to “lawful order[s]” and the fact that an order is unlawful can be a defense for refusing to obey. This does not mean that soldiers can employ their own judgment as to what is legal or not in deciding whether to follow or disregard orders, as there is a strong presumption in favor of the president’s interpretation of the law. But where soldiers are presented with an order that clearly runs contrary to widely understood legal limits—particularly where that understanding is backed up by an injunction or other judicial order—they may well have grounds (if not a legal and ethical obligation) to disobey.

The broad legal authority that presidents may have to authorize domestic deployments of the military is not a carte blanche as to how those military forces may be used. Absent meaningful reform, understanding these limitations—and being prepared to enforce them through the courts—may well be the best way to fight back against potential abuse.