So what would happen if, amid the panic of the coronavirus pandemic, the president tried to declare martial law? Without question, military forces directed by state governors—and perhaps even, in extreme cases, by the president—may be uniquely able to help get us through the current crisis. At least 20 state governors have now called up their National Guard to assist with delivery of food and medical supplies, clean public facilities, and adapt some of those facilities to house patients if hospitals become overwhelmed. Guard personnel could also help enforce quarantines ordered by state governors, and even arrest violators. But their role is to support, not replace, civil authorities. The states’ legal power to do all this is clear; it is not martial law.

Juliette Kayyem: Trump leaves states to fend for themselves

The fact is that there’s no guidance in the Constitution or the statutes, and only a limited historical record, to indicate what justifies a declaration of martial law. If martial law were invoked, the government would be conducted ad hoc by the president or a military commander based entirely on his or her opinion of what was needed to meet the emergency, unbound by any laws and with no transparency or public participation, and probably no accountability afterward. The result would be entirely unpredictable and unprincipled, a dangerous threat to American democracy.

The prospect of martial law, while still remote, is not purely hypothetical in the current crisis. Donald Trump has called himself a “wartime president.” His likely Democratic opponent in November, Joe Biden, describes efforts to stem the pandemic as “akin to war.” And last week Gavin Newsom, the governor of California, suggested that martial law is not necessary “at this moment”—implying that it may yet be considered. In a time of panic, statements like this naturally lead to questions about the authority of the president or state governors to deploy troops at home. Aware that the specter of martial law has arisen, Peter Gaynor, the head of FEMA, was at pains during the White House Coronavirus Task Force press conference on Sunday to emphasize that the use of the National Guard to help with the emergency “is not martial law.” And when Defense Secretary Mark Esper confirmed on Monday that President Trump had activated the National Guard in three states through Title 32—under which the state governors control the troops while the federal government pays for them—he emphasized that “this is not a move toward martial law, as some have claimed.”

The president’s powers to use federal troops (or National Guard forces called to active duty) are more limited than governors’. The president has announced that the Navy will use its two hospital ships to provide needed hospital beds, and the Defense Department has said that it will share critical medical supplies—ventilators, masks, and other protective gear—with civilian health-care workers. New York Governor Andrew Cuomo has also asked the Army Corps of Engineers to provide additional hospital beds for the acutely ill. What federal military forces may not legally do, however, is engage in law enforcement, except by following statutory procedures to suppress an insurrection, violent civil unrest, or an unlawful combination or conspiracy. The law says that job is left to state and local police and National Guard personnel, even in an emergency like the one we now face.