On Monday night, as federal law enforcement officers fired rubber bullets and chemical gas at protesters outside the White House, President Trump stood in the Rose Garden and issued a threat. If the nation’s governors don’t call up National Guard troops to “dominate the streets,” Trump said, “I will deploy the United States military and quickly solve the problem for them.” After Trump made the threat, three words began trending on Twitter: the Insurrection Act.

No, this is not the title of a Netflix series. It’s an actual law dating to 1807.

Originally signed by Thomas Jefferson and amended over the years, it states: “Whenever there is an insurrection in any State against its government, the President may, upon the request of its legislature or of its governor if the legislature cannot be convened,” send in military troops.

In case governors don’t comply with Trump’s demands, one provision of the law, passed in 1956, gives him the power to act unilaterally: “Whenever the President considers unlawful obstructions, combinations, or assemblages or rebellion against the authority of the United States, make it impracticable to enforce the laws … in any State … he may call into Federal service such of the militia of any State, and use such of the armed forces, as he considers necessary to enforce those laws or to suppress the rebellion.”

On Wednesday, Defense Secretary Mark T. Esper said he does not support using active-duty military forces to deal with the unrest in U.S. cities — a statement that put him at odds with his boss. Then Trump was denounced by his former Defense Secretary, Jim Mattis, for the threat to invoke the Insurrection Act and dividing Americans.

Over 200 years, the Insurrection Act has been used dozens of times, according to a Congressional Research Service report published in January.

The first time was by Jefferson just a year after he signed it into law. In 1808, Jefferson declared the Lake Champlain region in New York and Vermont to be “in a state of insurrection” based on its continued violations of the Embargo Act, according to a history of the law written by an Army major with the U.S. National Guard at the School of Advanced Military Studies. (Unlike Trump, Jefferson didn’t refer to the insurrectionists by derogatory names but instead called them “sundry persons.”)

In 1831, President Andrew Jackson, at the request of the Norfolk mayor, deployed federal troops to quash Nat Turner’s slave rebellion in Virginia’s Southampton County — the largest such revolt in U.S. history. A detachment of three army artillery companies arrived in Suffolk, Va., along with members of the Navy, according to the book “The Role of Federal Military Forces in Domestic Disorders, 1789-1878” by Robert W. Coakley.

Four decades later, in 1871, President Ulysses S. Grant expanded the Insurrection Act so that he could suppress uprisings against “any part of or class … of people” who are “deprived of a right, privilege, immunity, or protection named in the Constitution.”

Grant’s target was the violent Ku Klux Klan, which was, according to the Coakley book, inciting “its members to commit crimes, including murder, against Republicans and blacks” and rarely facing prosecution.

Once the law passed in the spring of 1871, Grant was quick to act. On May 3 of that year, he issued a required order for the scofflaws to cease violating anyone’s constitutional rights. Then, after learning that the KKK was still thriving in several South Carolina counties, he organized more than 1,000 soldiers to round up several hundred Klansmen. “Scores” of suspects were arrested, many of them interrogated for several weeks without an indictment. By Jan. 1, 1872, the Army had detained more than 600 men, and most of them were tried and convicted in federal court.

Over the next several decades, the law would be used many more times by other presidents. In 1957, President Dwight D. Eisenhower wielded the Insurrection Act during the desegregation of Central High School in Little Rock. Nine black students who enrolled at the all-white school were testing out the 1954 Supreme Court ruling in Brown v. Board of Education of Topeka, which concluded that segregated public schools were unconstitutional. On Sept. 25, Eisenhower sent federal troops to escort the students into the school safely, after they’d been barred earlier that month by National Guardsmen called in by the state’s governor.

More recently, President George H.W. Bush ordered federal troops into Los Angeles in 1992 after Peter Wilson, then-governor of California, made a request to help quell riots after the four police officers charged in the beating of Rodney King were acquitted.

In 2005, shortly after Hurricane Katrina devastated Louisiana and the Gulf Coast, President George W. Bush sought to put the state’s National Guard under federal control and take command of the state’s hurricane relief mission.

Looters and lawlessness reigned in the disaster’s aftermath, and White House lawyers wanted to investigate invoking the Insurrection Act regardless of whether the state’s governor approved. But, ultimately, Bush declined.

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