Recently, a rumor that President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE would impose martial law in response to the COVID-19 pandemic went viral. The story gained enough traction that the National Security Council stepped in to reassure the public it was fake. But Americans, used to dystopian films featuring government takeovers of quiet civilian life, still wonder: Could it happen here?

The answer is probably no — at least under circumstances as they now stand.

The president’s power to declare martial law is not nearly as broad as rumors suggest. The states’ powers are greater, but they too are subject to important restrictions.

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Nonetheless, uncertainties in the law show the need for Congress and state legislatures to clarify the scope and limits of martial law.

The concept of “martial law” is not well understood, let alone defined, in American law. It usually refers to military forces taking over the functions of ordinary civilian government. The key words are “taking over.” Although the military often provides support and assistance for certain activities performed by civilian authorities — such as carrying out search-and-rescue missions in the aftermath of a natural disaster — actual displacement of civilian government represents a dramatic departure from normal practice.

But it has happened.

There was a period in American history when martial law was relatively common. Between 1857 and 1945, martial law was declared 70 times in the United States. In most cases, a state governor imposed it on a city, county or group of counties. This was sometimes in response to violent civil unrest but more often to break strikes on behalf of business interests. These declarations lasted anywhere from days to years. Martial law was last declared in the United States in 1966, when the governor of California imposed it to suppress unrest in the Hunters Point neighborhood of San Francisco after a white police officer shot a black teenager.

The federal government has declared martial law too, though far less frequently. Most recently, Hawaii was placed under martial law for the majority of World War II. The U.S. Army controlled every aspect of civilian life on the islands, from criminal justice to curbside trash removal.

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The Supreme Court has addressed martial law in only a handful of cases and has offered little clear guidance about where the authority to declare it comes from. At the federal level, some scholars have argued that the president has inherent authority under the Constitution to declare martial law. The more persuasive view is that the president (or certain subordinates) can declare martial law only if Congress has authorized it. This was the case in Hawaii.

Because the power to declare martial law comes from Congress, Congress controls when, where and how it may be used. It can even take the power away. One limit Congress has placed on martial law — and on the domestic deployment of the military more broadly — is the Posse Comitatus Act. This law generally bars federal troops from participating in domestic law enforcement activities, such as arrests and detentions. It is not, however, an absolute prohibition on domestic military deployment.

The Posse Comitatus Act does not prevent federal troops from assisting civilian authorities in ways that do not involve law enforcement. Federal troops can help with a wide range of disaster response efforts without violating the act, such as when they were deployed to clean up debris and distribute relief supplies after Hurricane Katrina. More recently, they have been tasked with performing non-law enforcement duties in support of immigration enforcement at the southern border.

And several laws create exceptions to the Posse Comitatus Act’s general rule. The most important of these is the Insurrection Act. It allows the president, at the request of a state’s governor or legislature, to use federal troops to suppress an insurrection in that state. The president may also deploy troops without a state’s request if an “unlawful obstruction,” domestic violence or similar civil unrest is creating barriers to execution of the law. Troops may be used either to assist local law enforcement or, if necessary, to supplant them. In the latter case, the act authorizes a form of martial law, triggered by vague criteria and with few clear constraints on its use.

If violent riots were to break out across the United States due to panic over COVID-19, the president could deploy federal troops under the Insurrection Act to suppress them. But that seems unlikely. Even countries worst hit by the pandemic have not seen significant civil unrest. If mass riots were to take place, there would still be no need for federal troops to displace civilian authorities, as opposed to merely assisting them. In other words, there would be no legitimate reason to declare martial law.

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The legal framework changes when one shifts to the state level. The Posse Comitatus Act does not apply to the states’ use of their own National Guard forces. Whether a state can use its militia to assist with civilian law enforcement and whether it can go further and declare martial law depends on what its own constitution and laws allow. Many states’ laws authorize much broader use of the military than is permitted under federal law.

That does not mean governors can do whatever they want. The U.S. Constitution still applies under martial law. The states must comply with its requirements and with the requirements in their own constitutions. Although the Supreme Court has said that courts cannot second-guess whether a state governor’s declaration of martial law is necessary, they can review whether the actions taken under that declaration are constitutionally permissible.

This is also true at the federal level: The courts can review whether the military’s actions under martial law have crossed a constitutional line. The Supreme Court did exactly that in the case of Hawaii. Without setting aside the martial law declaration itself, the Court overturned the convictions of every civilian who had been tried by a military court while the islands were under martial law.

Judges typically allow the government greater flexibility in a crisis and will weigh the government’s interest in public health and safety heavily when balancing it against intrusions on civil liberties. That doesn’t mean those liberties disappear. It is unlikely, for example, that courts would uphold the use of the military to enforce quarantines only on certain ethnic groups.

While martial law is not the unbounded power that we often see portrayed in popular culture, the public fears it sparks highlight a real issue: The Supreme Court has left many questions about martial law unanswered, and the laws passed by Congress and the states leave too much room to exploit these uncertainties.

Once the immediate health crisis has passed, Congress and state legislatures should revisit these laws in order to clarify — and appropriately limit — when, where and how martial law may be used. Until this happens, rumors and misinformation about this frightening executive power will continue.

Joseph Nunn is a Fellow in the Brennan Center for Justice’s Liberty and National Security Program.