In the space of a few hours on Saturday, President Trump went from threatening an ALL-CAPS federal “QUARANTINE of developing ‘hot spots’, New York, New Jersey, and Connecticut” to… having CDC issue a “strong Travel Advisory” for the tristate area instead.

It’s hardly the first time this president has floated some eye‐​poppingly authoritarian proposal, only to back away from the Rubicon shortly thereafter. By now, the pattern should be familiar:

Trump hits “send tweet” on some crank theory of absolute executive power. Law professors and pundits cancel their weekend plans, scrambling to figure out “Can he do that?”—only to realize, weeks later, that they needn’t have taken him literally or seriously.

But when I wrote that in Reason magazine a year ago, “weekend plans” were still a thing. In the pre‐​COVID‐​19 era, it was usually safe to take the president’s autocratic reveries with a grain of salt. We’re in new territory now. The pressures for bold action in times of crisis have led far cooler‐​headed presidents to make desperate and dangerous moves, and there are far rougher days ahead. Our radically changed circumstances make this particular proposal worth worrying about.

A number of questions come to mind: first among them, “can he do that?”—is there any plausible legal authority for a domestic Travel Ban encircling three states and some 30 million Americans? Second, if he tried, how could the feds enforce it? And finally, just how nightmarish would that scenario be?

Let’s start with the legal question. The key statute for federal quarantine authority is the Public Health Service Act (PHSA), which has been used, in recent outbreaks, primarily for international travel restrictions. However, the PHSA also provides authority to issue regulations aimed at preventing disease spreading across state lines, and authorizes the apprehension and examination of

[A]ny individual reasonably believed to be infected with a communicable disease in a qualifying stage and (A) To be moving or about to move from a State to another State.…

The PHSA seems to envision individual testing, not interstate blockades. Regulations issued under its authority:

may provide that if upon examination any such individual is found to be infected, he may be detained for such time and in such manner as may be reasonably necessary.

That language does not appear to support a federal power to impose a cordon sanitaire around the Tristate Area unless virtually any person attempting to leave can be “reasonably believed” to already have COVID-19. Still, even more tortured interpretations of federal law have prevailed, at least temporarily, in past crises. I wouldn’t necessarily count on the courts to enjoin this one.

If the administration opted for an “enforceable quarantine,” how would the feds enforce it? “One option,” as George W. Bush put it in the wake of the Katrina debacle, “is the use of a military that’s able to plan and move.”

Indeed, for an operation on the scale Trump suggested, it may be the only option. “Neither the Public Health Services Act nor CDC regulations specifically authorize military enforcement of federal quarantine orders. But neither HHS nor CDC possess sufficient resources to enforce mass quarantines,” observes Jesse T. Greene in a 2015 Harvard National Security Journal article, “Federal Enforcement of Mass Involuntary Quarantines” (here’s hoping your social‐​distancing reading list consists of somewhat more soothing titles).

One potential obstacle to the use of federal troops for quarantine enforcement is the Posse Comitatus Act (PCA), the longstanding federal statute that makes it a criminal offense to use U.S. military forces to “execute the laws.” Still the PCA has never been an insurmountable barrier to using standing armies at home. By its terms, the statute doesn’t apply where Congress has elsewhere “expressly authorized” the use of federal troops to execute the laws. The PHSA doesn’t fit the bill, but, according to Greene and other analysts, another set of statutes might.

Those are the Insurrection Acts, based on Congress’s constitutional power to “To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions.” Those authorities were last invoked in 1992, when President George H.W. Bush sent in U.S. Marines and Army infantry to help quell the L.A. Riots.

Under 10 USC §252, Congress has delegated to the president the power to call out federal troops when “unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States, make it impracticable to enforce the laws of the United States in any State.” That language tracks with most uses of the Insurrection Acts historically, from the Whiskey Rebellion to the L.A. Riots. It’s a little tougher to see families attempting to cross state lines in the face of a federal quarantine as “unlawful combinations” or “rebellion against the authority of the United States.” The authority granted by the Insurrection Acts is “intentionally narrow and would not seem to allow federal troops to be used in a humanitarian situation like suppressing a pandemic,” Mark F. Cancian writes in a recent analysis for CSIS; still, “lawyers can be quite imaginative in finding ways to stretch statutory text.”

There’s good reason to resist that move. Soldiers are trained to be warriors, not peace officers — which is as it should be. But putting full‐​time warriors into a civilian policing situation can result in serious collateral damage to American life and liberty.

Greene, the author of the article on “Federal Enforcement of Mass Involuntary Quarantines,” supports military enforcement in certain circumstances. But he also acknowledges the grave risks entailed in the use of standing armies at home. He notes that the U.S. Coast Guard, which is not covered by the Posse Comitatus Act, “has organized to avoid problems like the one that occurred in 1992 when Marines accompanied LAPD officers.”

The Coast Guard has special units where Coast Guardsmen are trained to follow the “warrior mindset,” meaning they are focused on using overwhelming force. But, the Coast Guard never allows personnel in those special units to operate outside their units because “a 20 year old can’t just flick a switch in his head from warrior to enforcer mindset.”

Greene recounts an incident from the 1992 Marine deployment to Los Angeles.

Police officers responded to a domestic dispute, accompanied by marines. They had just gone up to the door when two shotgun birdshot rounds were fired through the door, hitting the officers. One yelled ‘cover me!’ to the marines, who then laid down a heavy base of fire .… The police officer had not meant ‘shoot’ when he yelled ‘cover me’ to the marines. [He] meant … point your weapons and be prepared to respond if necessary. However, the marines responded instantly in the precise way they had been trained, where ‘cover me’ means provide me with cover using firepower .… over two hundred bullets [were] fired into that house.

A few years later, a Marine Corps anti‐​drug patrol operating under the so‐​called drug war exceptions to the Posse Comitatus Act, shot and killed an 18‐​year‐​old American high school student who was herding goats near his family’s farm in Redford, Texas. An internal Pentagon investigation of the incident said that the soldiers were ill prepared for contact with civilians, as the Marines’ military training had instilled ‘‘an aggressive spirit while teaching basic combat skills.”

Worse still, Greene suggests that the military’s current Standing Rules for the Use of Force (SRUF) would exacerbate that problem, raising the risk of shoot‐​to‐​kill incidents in any federal quarantine. The SRUF may lead active‐​duty troops to believe they can use deadly force to stop Americans attempting to leave a quarantined area. To ameliorate that problem, Greene proposes replacing the current rules with revised SRUF designed to guard against lethal escalation “and ensure respect for the due process rights of quarantined persons.” But a better answer is to avoid such missions entirely.

Would interstate blockades enforced by federal troops do enough good to justify such a radical and risky departure from the American tradition of civilian law enforcement? How much, at this point, would they “flatten the curve”?

I’m not an epidemiologist, and I refrain even from playing one on Twitter. But actual experts in the containment of infectious diseases seem to recoil from Trump’s idea. The rush to get out of town before the clampdown “could end up creating more flight from New York and more chains of transmission,” argues Johns Hopkins’ Dr. Amesh Adalja. “The energy required to even begin to enforce something like that is probably better spent on core public health response activities,” echoes his colleague Joshua Sharfstein.

None of the above is to suggest that the US military shouldn’t help fight the COVID-19 pandemic. The Pentagon is the largest employer in the world and has manpower and assets it can bring to bear in this fight—without using soldiers against citizens.