Explainer: Trump keeps bringing up the Insurrection Act. What is it?

US President Donald Trump on Monday said he might use the Insurrection Act, a law authorizing the president to deploy military forces on US soil, if courts and governors continue to block his deployment of National Guard troops in Democratic-led cities.

Here's what to know about the centuries-old law.

WHAT IS THE INSURRECTION ACT?

The Insurrection Act is a federal law that gives the US president the power to deploy the military or federalize National Guard troops inside the US to quell domestic uprisings.

The law is commonly referred to as the Insurrection Act of 1807 because that was the year President Thomas Jefferson signed it into law. But the modern-day Insurrection Act is actually an amalgamation of statutes passed between 1792 and 1871 outlining the role of US military forces in domestic law enforcement.

The law enables troops to take part in domestic law enforcement activities like making arrests and performing searches, functions they are generally otherwise prohibited from engaging in.

The Insurrection Act can be invoked when there are "unlawful obstructions, combinations or assemblages or rebellion" against the US government's authority. When the president deems those conditions have been met, the law gives the president the power to use the armed forces to take actions "to enforce those laws or suppress the rebellion."

When Trump sent the National Guard to Los Angeles and other cities this summer, he relied on a different legal basis, Section 12406 of Title 10 of the US Code, that allows the president to bring the guard into federal service.

Unlike the Insurrection Act, Section 12406 prohibits the National Guard from carrying out civilian law enforcement activities. This means that they can help protect federal officers and property, but are not allowed to conduct arrests.

WHY IS THE INSURRECTION ACT SO CONTROVERSIAL?

There is a long American tradition of keeping the federal military out of civilian affairs.

The nation's founders, having witnessed abuses by the British military during colonial times, feared that giving the president unlimited control over troops would erode civil liberties and the democratic process, constitutional scholars have said. Under the US Constitution, governors generally have the authority to maintain order within state borders.

These principles are embodied in the Posse Comitatus Act, an 1878 law that generally barred the military from participating in civilian law enforcement activities. The Insurrection Act functions as a statutory exception to the Posse Comitatus Act.

Civil rights groups have long warned that the Insurrection Act gives the president broad authority to use the military as a domestic police force in ways the founding fathers did not intend.

WHAT HAS TRUMP SAID ABOUT THE INSURRECTION ACT?

Trump told reporters in the Oval Office on Monday that so far it has not been necessary to invoke the Insurrection Act but that he would consider doing so.

"We have an Insurrection Act for a reason," Trump said. "If people were being killed and courts were holding us up, or governors or mayors were holding us up, sure, I'd do that."

Trump, during his first presidential term, suggested he would use the Insurrection Act to quell unrest following the death of George Floyd, an unarmed Black man killed in police custody, but ultimately did not do so.

HAS THE INSURRECTION ACT BEEN USED BEFORE?

Yes. The Insurrection Act has been invoked on dozens of occasions throughout US history. Since the civil rights movement of the 1960s, however, its use has become "exceedingly rare," according to a report by the Congressional Research Service.

The Insurrection Act was last used in 1992, when the acquittal of four Los Angeles police officers in the beating of Black motorist Rodney King led to deadly riots. California's governor had requested military aid from President George H.W. Bush to quell the violence.

CAN TRUMP SEND IN TROOPS WITHOUT A GOVERNOR'S APPROVAL?

Yes. The law lays out a scenario in which the president is required to have approval from a state's governor or legislature, and also instances where such approval is not necessary.

Historically, in instances where the Insurrection Act was invoked, presidents and governors have usually agreed on the need for troops.

In 2005, former President George W. Bush decided not to invoke the Insurrection Act to send active-duty troops to Louisiana in the aftermath of Hurricane Katrina, in part because the state's then-governor opposed the move.

CAN A COURT STRIKE DOWN TRUMP'S APPLICATION OF THE LAW?

Courts have historically been very reluctant to second-guess a president's military declarations, and the 9th US Circuit Court of Appeals recently said that the president's decision to send in the military is entitled to "great level of deference."

But some legal experts have argued that this deference does not completely stop courts from reviewing the president's decisions. An Oregon federal judge recently ruled against Trump's decision to send troops to protests in Portland, Oregon by invoking Section 12406, writing that "'a great level of deference' is not equivalent to ignoring the facts on the ground."