What is a president's authority to send in federal troops?

President Donald Trump has vowed to use the U.S. military to quell the riots and unrest across the country in the wake of George Floyd’s death if cities or states “refuse to take the actions that are necessary to defend the life and property of their residents.”

But what is the president’s authority to send in troops?

The Constitution says Congress has the power “to provide for the calling forth of the militia … to suppress insurrections,” and it has given that authority to the president in various forms since 1792.

As currently worded, the Insurrection Act allows the president to call up the active military or federalize the national guard under three circumstances:

• At the request of a state. That’s how it was most recently used, when then-California Gov. Pete Wilson asked for federal help in 1992 to control violent protests after police officers were acquitted in the attack on Rodney King.
• To enforce federal law. In 1987, President Ronald Reagan ordered the Pentagon to provide military units to help suppress violence at a federal prison in Atlanta. The disturbance was over before the troops arrived.
• To protect civil rights. This provision authorizes the president to use the military to suppress “any insurrection, domestic violence, unlawful combination, or conspiracy” if local law enforcement is unable to provide sufficient protection. It does not require the request — or even the permission — of the state. President Dwight D. Eisenhower used this power to send elements of the 101st Airborne to Little Rock and to federalize the entire Arkansas National Guard to enforce court-ordered school desegregation. Presidents John F. Kennedy and Lyndon Johnson invoked the same authority to enforce other desegregation orders in Mississippi and Alabama.

After Hurricane Katrina, Congress changed the law, allowing the president to use this provision in cases of domestic violence when public order is disrupted by natural disaster, epidemic, or terrorist attack, without a request from the state governor. But Congress revoked that specific authority a year later in the face of widespread opposition from the states.

So what about now? Can Trump send federal troops to a state that doesn’t ask for them or even opposes them? The current law doesn’t explicitly allow it. But it doesn’t clearly forbid it, either, and history is full of examples where presidents broadly interpreted this law or its forerunners.

Even so, an administration official said Tuesday that unless conditions substantially deteriorate, there are no current plans to send federal troops to a state against a governor’s wishes.

CORRECTION (June 2, 2020, 2:18 p.m. ET): An earlier version of this article misstated that Rodney King had died in a police beating and that four police officers were acquitted in his death. King survived the 1992 police beating, dying in 2012, and the four officers were acquitted of excessive use of force.