What is the Insurrection Act, and what happens if Trump uses it to quell LA protests?

Protests in Los Angeles are entering their fourth day over the Trump administration's immigration crackdown.

President Donald Trump, over the weekend, called protesters "violent, insurrectionist mobs" after he deployed the National Guard despite objections from California's Democratic Gov. Gavin Newsom.

The escalatory step is prompting a host of legal questions, including how far Trump is willing to go to use his authority to curb protests over his administration's immigration raids.

On Sunday, Trump was asked by ABC News Senior Political Correspondent Rachel Scott if he is prepared to invoke the 1807 Insurrection Act. The last time the act was used was in 1992 during the Los Angeles riots.

"Depends on whether or not there is an insurrection," Trump replied.

When asked by Scott if he thought an insurrection was taking place in Los Angeles, Trump replied, "No, no. But you have violent people, and we are not going to let them get away with it," Trump said at the time. But by Sunday night, he was referring to the protesters on his Truth Social platform as "violent, insurrectionist mobs" and "paid insurrectionists."

Asked to define insurrection, Trump said, "You actually really just have to look at the site to see what's happening."

Trump notably did not rule out sending active-duty Marines to California after Defense Secretary Pete Hegseth said they were standing by. A U.S. official confirmed on Monday afternoon that 700 Marines from Twentynine Palms, California, have been ordered to assist in Los Angeles.

The bar for sending active-duty Marines? "The bar is what I think is," Trump had said on Sunday.

What to know about the Insurrection Act

Generally, the use of federal troops on U.S. soil is mostly prohibited. The 1878 Posse Comitatus Act limits the military from being involved in civilian law enforcement unless Congress approves it or under circumstances "expressly authorized by the Constitution."

One exception is the Insurrection Act, a 218-year-old law signed by President Thomas Jefferson.

The Insurrection Act states, in part: "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, call into Federal service such of the militia of the other States, in the number requested by that State, and use such of the armed forces, as he considers necessary to suppress the insurrection."

Another provision states it can be used "whenever the President considers that unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States, make it impracticable to enforce the laws of the United States in any State by the ordinary course of judicial proceedings."

Some legal experts have warned the law is overly broad and vague, and there have been various calls for it to be reformed to provide greater checks on presidential power.

The Insurrection Act has been invoked in response to 30 crises over its history, according to the Brennan Center for Justice, including by presidents Dwight D. Eisenhower and John F. Kennedy to desegregate schools after the Supreme Court's landmark ruling in Brown v. Board of Education.

Most of its uses involved federal troops being deployed, though a few situations were resolved after troops were ordered to respond but before they arrived on the scene, the Brennan Center noted.

When it was last used in 1992 by President George H.W. Bush to send the National Guard to Los Angeles, it was at the request of then-Gov. Pete Wilson as riots exploded in the city after the acquittal of four white police officers charged in the beating of Rodney King.

If Trump were to invoke the act, he would likely be doing so against Newsom's wishes -- something that hasn't been done since President Lyndon B. Johnson in the 1960s to deal with civil unrest.

How Trump mobilized the National Guard

Trump did not invoke the Insurrection Act when he activated and deployed the National Guard to Los Angeles.

Instead, he cited Title 10 of the U.S. Code -- which contains a provision that allows the president to call on federal service members when there "is a rebellion or danger of rebellion against the authority of the Government of the United States" or when "the President is unable with the regular forces to execute the laws of the United States."

According to a presidential memorandum, Trump said he was sending the National Guard to "temporarily protect ICE and other United States Government personnel who are performing Federal functions, including the enforcement of Federal law, and to protect Federal property, at locations where protests against these functions are occurring or are likely to occur based on current threat assessments and planned operations."

The memo stated that 2,000 National Guard troops could be deployed for 60 days or "at the discretion" of Hegseth.

Troops called up under Title 10 fall generally are prevented from direct involvement in law enforcement duties under the Posse Comitatus Act, unless Trump invokes the Insurrection Act or other limited exceptions apply.

Newsom said on Monday the state is suing the administration over Trump deploying the National Guard.

"He flamed the fires and illegally acted to federalize the National Guard," Newsom wrote on social media. "The order he signed doesn't just apply to CA. It will allow him to go into ANY STATE and do the same thing. We're suing him."