What Is the Insurrection Act And Does it Give Trump the Authority to Send Military Troops Into States? Here’s What To Know

President Donald Trump on Monday threatened to send the United States military into states if the unrest that has swept multiple U.S. cities continues to grow, citing a 213-year-old law, the Insurrection Act of 1807, as his legal authority.

“If a city or state refuses to take the actions that are 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,” the President said Monday evening. He announced he had already authorized thousands of troops to deploy to Washington D.C. in response to days of increasing unrest in the city.

Over the past week, protests in response to the murder of George Floyd — a 46-year-old black man who was killed in police custody in Minneapolis on May 25 — have swept the U.S.. While most protests have been peaceful, some demonstrations have grown violent, with police turning rubber bullets and tear gas on crowds and some protesters setting property on fire. Governors have already activated over 17,000 National Guard members in at least 23 states, according to the National Guard Bureau. The National Guard has also been deployed to Washington D.C., in addition to the military troops who arrived in the capital city Monday night.

Washington D.C. is a federal district, meaning the president has authority to deploy troops there if he chooses. However, deploying troops to the rest of the U.S. is less simple. The Posse Comitatus Act of 1978 says the military cannot enforce law in U.S. states or territories without the express authorization of Congress. However, the Insurrection Act, which was passed by Congress and is itself an express authorization, provides an exception, legal experts tell TIME.

“There’s this long tradition of not wanting the military to be used to enforce federal law, or federal Constitutional rights,” says Saikrishna Prakash, a professor of law at the University of Virginia Law School whose work focuses on executive power. “But there’s also been a long tradition of it actually being used.” The Insurrection Act was invoked multiple times to enforce desegregation during the Civil Rights Movement, for example.

Here’s what to know about the Insurrection Act and the power it gives the president.

What is the Insurrection Act of 1807?

The Insurrection Act of 1807 gives the President the power to deploy the National Guard or the military to enforce laws in certain circumstances. It expanded upon the Militia Act of 1792, which gave the president power to command state militias in cases of an insurrection or an invasion “from any foreign nation or Indian tribe.”

The Insurrection Act can be invoked if there’s an insurrection against state law and a state government requests federal assistance restoring order, says Daniel Hulsebosch, a professor of law at New York University School of Law who specializes in early U.S. legal history.

It can also be invoked if there’s an insurrection against federal law, Hulsebosch continues. Crucially, after the Civil War, Congress added a provision allowing the president to invoke the Act without a state’s permission if the state is failing to protect the Constitutional rights of its citizens.

Presidents have invoked the Insurrection Act dozens of times throughout U.S. history, according to a report by the Congressional Research Service. It was invoked numerous times in the middle of the 20th century to enforce desegregation and respond to riots. For example, President Lyndon Johnson invoked the Act to deploy federal troops to Detroit in response to the 1967 riot.

Use of the act dropped off after the 1960s. The last time it was used was in 1992, during the Los Angeles riots in response to the acquittal of four white police officers who had been charged with the beating of black motorist Rodney King. (Some have compared the 1992 riots to the protests against police brutality that swept the country last week.) In that instance, California’s Governor Pete Wilson requested federal assistance suppressing the riots.

But the Act has also been invoked without a state’s permission in the past. For example, President Dwight Eisenhower invoked the Act in 1957 to send the 101st Airborne Division to Little Rock, Ark., to maintain order during the integration of Central High School, against the wishes of Arkansas’ governor.

Can Trump use the Insurrection Act to send military troops into states to put down protests?

He can, says Prakash, although he must meet certain legal qualifications.

The President could invoke the Insurrection Act if a state requests help enforcing state law, but so far no state has done so. He could invoke the Act to enforce federal law, but he first must issue a proclamation ordering those violating the law to disperse. If they do not disperse, he could then order either the military or the state’s National Guard to go in and suppress what he would be arguing are “unlawful obstructions, combinations or assemblages against the authority of the United States,” says Prakash.

If a governor forbids Trump from sending troops into their state — which Democratic governors J. B. Pritzker of Illinois and Andrew Cuomo of New York have already done — the President could still send in troops if he argues federal law is being obstructed or that the state isn’t protecting the rights of its citizens, explains Prakash. If protests are peaceful, Prakash says there’ll likely be no legal basis for the Insurrection Act. But reports that a small portion of the protesters are engaging in looting and arson could possibly be the basis of the President’s invocation of the Act, he explains. The President could try and argue that states are denying equal protection of the laws to citizens whose property is being burned or stolen, says Prakash.

President George H. W. Bush also already invoked the Insurrection Act in response to looting in 1989, when he sent troops to the U.S. Virgin islands after looting broke out following the devastation of Hurricane Hugo, according to NPR. Current Attorney General Bill Barr was also Attorney General under President Bush at that time.

If President Trump invokes the Insurrection Act, the move would likely be challenged in court, says Prakash, who adds that he wouldn’t be surprised if “some district court somewhere said [the move] was unlawful.” But, in Prakash’s opinion, the invocation would probably ultimately be upheld. However, Prakash points out that even if the President eventually won the legal battle, an injunction could delay troop deployment and limit the President’s ability to send in troops to the current protests.