Donald Trump would not be the first president to invoke the Insurrection Act, as he has threatened, so that he can send US military forces to Minnesota.
But he'd be the only commander-in-chief to use the 19th-century law to send troops to quell protests that started because of federal officers the president already has sent to the area – one of whom shot and killed a US citizen.
The law, which allows presidents to use the military domestically, has been invoked on more than two dozen occasions – but rarely since the 20th Century's Civil Rights Movement.
Federal forces typically are called to quell widespread violence that has broken out on the local level – before Washington's involvement and when local authorities ask for help. When presidents acted without local requests, it was usually to enforce the rights of individuals who were being threatened or not protected by state and local governments. A third scenario is an outright insurrection – like the Confederacy during the Civil War.
Experts in constitutional and military law say none of that clearly applies in Minneapolis.
"This would be a flagrant abuse of the Insurrection Act in a way that we've never seen," said Joseph Nunn, an attorney at the Brennan Center for Justice's Liberty and National Security Program. "None of the criteria have been met."
William Banks, a Syracuse University professor emeritus who has written extensively on the domestic use of the military, said the situation is "a historical outlier" because the violence Trump wants to end "is being created by the federal civilian officers" he sent there.
But he also cautioned Minnesota officials would have "a tough argument to win" in court because the judiciary is hesitant to challenge "because the courts are typically going to defer to the president" on his military decisions.
Here is a look at the law, how it's been used and comparisons to Minneapolis.
George Washington signed the first version in 1792, authorising him to mobilise state militias – National Guard forerunners – when "laws of the United States shall be opposed, or the execution thereof obstructed."
He and John Adams used it to quash citizen uprisings against taxes, including liquor levies and property taxes that were deemed essential to the young republic's survival.
Congress expanded the law in 1807, restating presidential authority to counter "insurrection or obstruction" of laws. Nunn said the early statutes recognised a fundamental "Anglo-American tradition against military intervention in civilian affairs" except "as a tool of last resort".
The president argues Minnesota officials and citizens are impeding US law by protesting his agenda and the presence of US Immigration and Customs Enforcement officers and Customs and Border Protection officers. Yet early statutes also defined circumstances for the law as unrest "too powerful to be suppressed by the ordinary course" of law enforcement.
There are between 2,000 and 3,000 federal authorities in the Minneapolis-St. Paul metro area, compared to Minneapolis, which has fewer than 600 police officers. Protesters' and bystanders' video, meanwhile, has shown violence initiated by federal officers, with the interactions growing more frequent since Renee Good was shot three times and killed.
"ICE has the legal authority to enforce federal immigration laws," Nunn said. "But what they're doing is a sort of lawless, violent behaviour" that goes beyond their legal function and "foments the situation" Trump wants to suppress.
"They can't intentionally create a crisis, then turn around to do a crackdown," he said, adding that the constitutional requirement for a president to "faithfully execute the laws" means Trump must wield his power, on immigration and the Insurrection Act, "in good faith".
Courts have blocked some of Trump's efforts to deploy the National Guard, but he'd argue with the Insurrection Act that he does not need a state's permission to send troops.
That traces to President Abraham Lincoln, who held in 1861 that Southern states could not legitimately secede. So, he convinced Congress to give him express power to deploy U.S. troops, without asking, into Confederate states he contended were still in the Union. Quite literally, Lincoln used the act as a legal basis to fight the Civil War.
Nunn said situations beyond such a clear insurrection as the Confederacy still require a local request or another trigger that Congress added after the Civil War: protecting individual rights. Ulysses S. Grant used that provision to send troops to counter the Ku Klux Klan and other white supremacists who ignored the 14th and 15th amendments and civil rights statutes.
During post-war industrialisation, violence erupted around strikes and expanding immigration – and governors sought help.
President Rutherford B Hayes granted state requests during the Great Railroad Strike of 1877 after striking workers, state forces and local police clashed, leading to dozens of deaths. Grover Cleveland granted a Washington state governor's request – at that time it was a US territory – to help protect Chinese citizens who were being attacked by white rioters. President Woodrow Wilson sent troops to Colorado in 1914 amid a coal strike after workers were killed.
Federal troops helped defuse each situation.
Banks stressed that the law then and now presumes that federal resources are needed only when state and local authorities are overwhelmed – and Minnesota leaders say their cities would be stable and safe if Trump's feds left.
As Grant had done, mid-20th-century presidents used the act to counter white supremacists.
Franklin Roosevelt dispatched 6,000 troops to Detroit – more than double the US forces in Minneapolis – after race riots that started with whites attacking Black residents. State officials asked for FDR's aid after riots escalated, in part, Nunn said, because white local law enforcement joined in violence against Black residents. Federal troops calmed the city after dozens of deaths, including 17 Black residents killed by local police.
Once the Civil Rights Movement began, presidents sent authorities to Southern states without requests or permission, because local authorities defied US civil rights law and fomented violence themselves.
Dwight Eisenhower enforced integration at Central High School in Little Rock, Arkansas; John F Kennedy sent troops to the University of Mississippi after riots over James Meredith's admission and then preemptively to ensure no violence upon George Wallace's "Stand in the Schoolhouse Door" to protest the University of Alabama's integration.
"There could have been significant loss of life from the rioters" in Mississippi, Nunn said.
Lyndon Johnson protected the 1965 Voting Rights March from Selma to Montgomery after Wallace's troopers attacked marchers' on their first peaceful attempt.
Johnson also sent troops to multiple US cities in 1967 and 1968 after clashes between residents and police escalated. The same thing happened in Los Angeles in 1992, the last time the Insurrection Act was invoked.
Riots erupted after a jury failed to convict four white police officers of excessive use of force despite video showing them beating Rodney King, a Black man. California Gov. Pete Wilson asked President George HW Bush for support.
Bush authorised about 4,000 troops – but after he had publicly expressed displeasure over the trial verdict. He promised to "restore order" yet directed the Justice Department to open a civil rights investigation, and two of the LA officers were later convicted in federal court.
