Last night in a nationally televised speech, President Trump charged that those involved in the past week’s post-protest looting and arson were committing “acts of domestic terror.” Further, Trump threatened that “if a city or state refuses to take the actions necessary to defend the life and property of their residents, then I will deploy the United States military and quickly solve the problem for them.”

His address came after a week of mass protests about the death of George Floyd, who died after a Minneapolis police officer knelt on his neck for eight minutes while Floyd lay on the ground, saying he couldn’t breathe. Some protests have been followed by chaos, with angry individuals setting fires, throwing rocks, and looting. In some cities, police marched with protesters or expressed horror at Floyd’s death; in others, police escalated the violence.

Now the Trump administration appears likely to call in military force.

Who can decide when to send in the National Guard or the U.S. military? What happens then? Here’s what you need to know.

What are governors and the president doing now?

In nearly half the states, governors have called out the National Guard. The president has called up National Guard forces from several states to deploy in Washington, D.C. Minnesota Gov. Tim Walz (D) has fully mobilized the Minnesota Guard, is seeking assistance from neighboring states and has asked the Pentagon to be ready to send federal military help.

The Department of Defense has put several military police units on higher alert status, but as yet no governor has requested federal aid. While most regular military forces have little training for civil disturbance operations, military police are fairly well-prepared for them. Regular military forces prefer, however, to leave such operations to the National Guard, for whom response to civil disturbance is a core mission.

Who has the authority to use the National Guard or the regular military?

Depending on the state constitution, a governor may have sole authority or may need the state legislature’s permission to call up the state’s National Guard to help enforce the law and maintain public order. State laws lay out specific rules for how the Guard may be used, but most give governors broad authority and discretion.

When the National Guard is called up under state authority, it is under the governor’s command and paid for out of the state budget. The president can elect to put Guard forces under federal authority under Title 32 of the U.S. Code while leaving them under state command; that lets the federal government pick up costs, as it has done to help support states’ covid-19 responses.

The president can also fully federalize the Guard under Title 10, which brings the force under the president’s control through the secretary of defense, with the federal government picking up the whole tab. That’s used mainly when the federal government wants to deploy Guard units outside their home states or use a state’s Guard against its governor’s wishes, as President Dwight Eisenhower did during the 1957 desegregation of Central High School in Little Rock, Ark.

Governors have no authority to call directly upon regular federal military forces for aid, but governors or state legislatures can ask the president to send in federal forces.

When does the president have the authority to either federalize the National Guard or deploy regular federal troops? According to the 1807 Insurrection Act, those situations include protecting federal property; enforcing federal law, such as civil rights; putting down an insurrection that threatens federal laws or rights, whether or not a state has requested the help; or if a state government asks, to help civil authorities manage civil disturbances that threaten that state’s laws or functioning.

If the president chooses to send federal forces over a state government’s objections, the state has few options for remedy.

What can the U.S. military do on domestic soil?

The Defense Department’s Directives/Instructions 3025.18 and 3025.21 lay out the rules under which regular federal forces can operate within the United States. As the instructions note, however, restrictions on the domestic use of military force apply only when those uses are not otherwise authorized by law. As legal scholar Steve Vladeck has shown, that exception is very broad. Normally, military units cannot arrest civilians or participate in searches and seizures. But if the president lawfully orders them to enforce law, aid civil authorities or suppress an insurrection, the normal restrictions do not apply.

How does the military behave when used within the U.S.?

Since the nation’s founding, both the National Guard and U.S. federal forces have often been called upon to restore order during civil disturbances. Sometimes this goes well, and sometimes very poorly. When military forces approach situations with the aim of de-escalating, unrest may die down without further violence. For instance, in 1894, Maj. Gen. John Schofield was ordered to halt more than 600 protesters who had hijacked a train in Montana and overpowered a group of U.S. marshals. He had his troops surround them while they were sleeping but insisted that they not be harassed. The protesters did not resist and were sent on with the marshals to Helena.

But when military forces act aggressively and take clear sides, they exacerbate the tensions that caused unrest in the first place. In Elaine, Ark., in 1919, white mobs massacred black residents. While the white mobs dispersed when federal troops arrived, it was largely because those troops took their side, arresting hundreds of black citizens and placing them in stockades for days. Some troops reportedly tortured or even killed prisoners.

The president’s language signals an aggressive and escalatory posture, designed to restore “law and order.” As always, the question is: What kind of order will be imposed, on whom?

Lindsay P. Cohn (@lindsaypcohn) is an associate professor in the Department of National Security Affairs at the U.S. Naval War College, currently researching domestic use of the U.S. military for law enforcement and peacekeeping operations. The views given here are her own and do not represent the views of the U.S. Naval War College or any organ of the U.S. government.