President TrumpDonald John TrumpSessions accepts ‘Fox News Sunday’ invitation to debate, Tuberville declinesPriest among those police cleared from St. John’s Church patio for Trump visitTrump criticizes CNN on split-screen audio of Rose Garden address, protesters clashing with policeMORE is threatening to deploy U.S. troops to states and cities that dont crack down on destructive protests that have gripped the nation amid the uproar over the killing of an unarmed black man in police custody.

In order to send troops to police the streets, Trump would need to invoke an 1807 law known as the Insurrection Act, which authorizes the commander-in-chief to deploy active-duty troops within the United States to enforce federal or state laws under certain circumstances.

Here are five things to know about the statute.

Trump enjoys broad discretion to dispatch troops under the law

The Insurrection Act allows presidents to deploy troops to hot spots across the country, a sort of carve-out to the general prohibition against using the military to enforce domestic laws.

Troops can be dispatched to cities and states at a governor’s request. But the law also gives presidents the authority to do so on their own to enforce federal or state laws, protect civil rights or quell insurrection.

What it is is that the president can deploy federal troops if in his judgment there is domestic violence or unlawful gatherings that interfere with the execution of federal or state law, said Elizabeth Goitien, co-director of the Brennan Center’s Liberty and National Security Program.

Legal experts say political pushback and even court challenges could follow if Trump were to make such a move without a states invitation.

The governors may challenge him in court and he may pay at the ballot box, said Thaddeus Hoffmeister, a law professor at the University of Dayton. Arguably, a citizen who was directly impacted by the order may also bring an action in court against the order.”

New York Attorney General Letita James tweeted that the state will not hesitate to go to court to protect our constitutional rights during this time & well into the future. New York has witnessed some of the most violent protests following the death of George Floyd, who was killed in Minneapolis police custody when a white police officer knelt on his neck for roughly eight minutes.

New York Gov. Andrew CuomoAndrew CuomoNew York curfew moved up to 8 pmRudy Giuliani calls on Cuomo to remove Bill de BlasioAmash readying legislation allowing victims to sue officersMORE (D) said thank you but no thank you to the presidents suggestion he would dispatch the military.

However, the broad discretion Trump enjoys under the act likely puts him on firm legal footing.

What I think [governors] could do was file a lawsuit arguing the president has overstepped those boundaries or misapplied the law, said Elie Honig, a legal analyst and former federal prosecutor. That is a serious uphill climb.

Trump has to formally invoke it

Trump has threatened to deploy troops to U.S. cities and states if governors do not meet his demands to activate sufficient numbers of National Guardsmen to contain protests, but he hasnt formally invoked the Insurrection Act.

I have strongly recommended to every governor to deploy the National Guard in sufficient numbers that we dominate the streets. Mayors and governors must establish an overwhelming law enforcement presence until the violence has been quelled, Trump said during a brief Rose Garden address Monday evening.

If a city or a 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 continued.

Trump would need to issue a formal proclamation in order to dispatch troops, according to Steven Vladeck, a University of Texas law professor. The president did not offer a timeline on potential decisions to deploy troops during Mondays address.

The National Guard has already been activated in 28 states and Washington, D.C., and the Trump administration has put active duty troops in the capital region on standby. On Monday evening, before he delivered his address, protesters were cleared from Lafayette Park across from the White House by law enforcement using tear gas ahead of a citywide 7 p.m. curfew.

We are putting everybody on warning: Our seven oclock curfew will be strictly enforced, Trump said. Those who threaten innocent life and property will be arrested, detained, and prosecuted to the fullest extent of the law.

Military force against citizens

When confronting civil unrest, U.S. armed forces are bound by guidelines known as the Standing Rules for the use of Force.

Those rules do not appear in the Insurrection Act. Rather, theyre developed on a case-by-case basis by the chairman of the Joint Chiefs of Staff, tailored to each domestic operation. For instance, when U.S. troops partner with a states National Guard, use-of-force rules are crafted with that states laws in mind, according to a U.S. Army handbook for military lawyers.

Domestic rules of engagement tend to be stricter than those for overseas missions, though there is some overlap, like permitting armed force only as a last resort, according to Mark Nevitt, a professor at the U.S. Naval Academy.

That means if a U.S. military member were to adhere literally to Trumps controversial invocation of the phrase, when the looting starts, the shooting starts, Nevitt wrote, it would mark a clear violation of the rules.

Using deadly force aggressively to stop looting clearly violates the governing rules for the use of force, principles of de-escalation, and the principles of using only minimum force, as a last resort, Nevitt wrote for the national security outlet Just Security.

But the rules slacken in other contexts.

They do not limit force undertaken in self-defense, and they also allow more latitude for defending vital national security facilities, weapons sites or critical infrastructure, the Army handbook states.

Military forces have fundamentally different training and mission than police, Goitien said. Theres a real risk that youre going to see violent clashes with civilians increasing when you put soldiers into the scene.

The U.S. military generally follows the lead of local police and state National Guard, though troops are able to conduct arrests and searches, subject to certain limitations.

Rare use in history

The law has been used in rare and extreme cases throughout U.S. history.

The Insurrection Act was invoked a number of times during the 1950s and 1960s to send troops to enforce federal civil rights law.

Presidents Dwight D. Eisenhower and John F. Kennedy drew on it to enforce the racial desegregation of Southern schools after the Supreme Courts 1954 decision in Brown v. Board of Education. The moves by Eisenhower and Kennedy came over the objections of state governors.

Eisenhower sent troops to Little Rock and Kennedy to Mississippi, where they were not requested or welcome, said Barbara McQuade, a law professor at the University of Michigan.

Most recently, it was invoked by former President George H.W. Bush in 1992 to quell riots in Los Angeles following the acquittal of police officers involved in the beating of Rodney King. At the time, Attorney General William BarrWilliam Pelham BarrTrump praises ‘domination’ of DC protestersAntifa and anarchists have hijacked Floyd protests, but the left won’t admit itThe Hill’s Morning Report – Trump mobilizes military against ‘angry mob,’ holds controversial photo opMORE was serving his first stint atop the Justice Department.

The law, which has been amended a number of times, was revised in 2006 after Hurricane Katrina to allow for its use in a broader range of public safety situations, but those changes were repealed the following year.

Some Republicans have advocated for its use amid growing unrest

A handful of Republicans have advocated for the use of the U.S. military to support local law enforcement in containing the unruly protests.

Sen. Tom CottonThomas (Tom) Bryant CottonThe Memo: Trump lags in polls as crises pressOvernight Defense: Esper, Milley part of ‘command center’ for response to protests over George Floyd killing | Several West Point cadets test positive for coronavirus ahead of Trump commencement speech | UN report says Taliban, al Qaeda not breaking tiesThe Hill’s Coronavirus Report: Former Rep. Delaney says Trump is spewing venom when he should be leading; Protests roil the nation as fears of new virus outbreaks grow MORE (R-Ark.) raised the idea of Trump invoking the Insurrection Act during an interview Monday morning on Fox News, hours before Trump delivered his Rose Garden address.

“The rioting, the anarchy and the looting ends tonight. If local law enforcement is overwhelmed … lets see how these anarchists respond when the 101st Airborne is on the other side of the street, Cotton said.

Trump later tweeted that Cottons remarks were 100% Correct.

Sen. Marsha BlackburnMarsha BlackburnBipartisan senators call for investigation of TikTok’s child privacy policiesSunday shows preview: States begin to reopen even as some areas in US see case counts increaseCommerce Department cracks down on Huawei’s access to chipsMORE (R-Tenn.) also tweeted that the president should send in the troops if big city mayors cant restore order.

Democrats, meanwhile, have criticized the presidents rhetoric and actions, arguing Trump is contributing to the upheaval. Many have zeroed in on the decision to clear protesters in the nations capital with tear gas.

“We would hope that the president of the United States would follow the lead of so many other presidents before him to be a healer-in-chief, and not a fanner of the flame,” Speaker Nancy PelosiNancy PelosiTrump praises ‘domination’ of DC protestersPelosi, Schumer say treatment of protesters outside White House ‘dishonors every value that faith teaches us’Democrats call for Congress to take action following death of George FloydMORE (D-Calif.) said in remarks Tuesday.