For the first time since 1992, the US Marine Corps are being mobilised by the military to respond to street protests in the United States.
Tensions have been rising since President Donald Trump activated the National Guard on Saturday after protests erupted in response to immigration raids in Southern California.
The decision to deploy about 700 marines to Los Angeles has been described as a further escalation.
Although some marines receive training in riot response, retired military officers say it is a drastic move that raises concerns.
They say it was delivering a message that may not be entirely peaceful.
Why are the marines going to LA?
Thousands of demonstrators have taken to the streets during the past four days, many gathering outside a federal detention centre in downtown Los Angeles where immigrants have been held.
Although the scenes have been dramatic, so far, the protests have only resulted in a few dozen arrests and some property damage.
California Governor Gavin Newsom has insisted the situation was "under control", slamming Mr Trump's decision to send an additional 2,000 National Guard troops along with hundreds of Marines.
"Trump is trying to provoke chaos," he wrote on X.
"The absurdity of threatening the people of the United States of America with their own military is morally reprehensible."
US officials said the marine troops were being deployed to protect federal property and personnel, including federal immigration agents.
"Due to increased threats to federal law enforcement officers and federal buildings, approximately 700 active-duty US Marines from Camp Pendleton are being deployed to Los Angeles to restore order," defence secretary Peter Hegseth wrote on X.
"We have an obligation to defend federal law enforcement officers — even if Gavin Newsom will not."
Former US Marine Corps officer Gary Barthel, managing partner of the Military Law Center in California, said it was unusual to send the marines when a situation had not been officially deemed an insurrection.
Since the protests first broke out, Mr Trump and his aides have repeatedly used the term "insurrectionists" to describe the demonstrators.
But troops would not be able to directly participate in civilian law enforcement unless Mr Trump invokes the Insurrection Act.
"They would likely be playing a similar role to the National Guard, controlling crowds or mobs, protecting or setting up barriers to certain federal properties, or keeping the riot condensed to a certain area," General Barthel told the ABC.
"But I think it is a drastic response. The Insurrection Act has not been enacted, and so this step is out of the ordinary."
What could the marines do?
The troops being deployed are coming from Twentynine Palms, which is a premier site for combat training in the desert.
General Barthel said they might be used so they could bring in personnel carriers to help block off roads or safely transport troops to different areas.
That may include light armoured vehicles that hold up to 20 marines.
"If you have vehicles burning on the street or whatever, they can utilise those vehicles to get through rough areas to drop the marines off where they need to be deployed," he said.
He added that it was likely the troops could be armed, but there would be "very strict policies" around the use of deadly force.
"More likely they would be using non-lethal force such as rubber bullets or something else, if need be," he said.
The marines being activated are with 2nd battalion, 7th Marines, 1st Marine division, according to US Northern Command.
Retired Lieutenant Colonel Russell Worth Parker said the marines have a large scope.
They were generally given training in riot control, but one of their major roles was as "shoot troops".
"We say every marine is a rifleman because every marine is trained to perform that mission to an extent. But not every marine does that for a living," he said.
The unit being sent was an infantry battalion, which was "a combat unit by definition", he added.
They were capable of non-combat operations and trained for those scenarios, but were also trained to "locate, close with, and destroy the enemy".
"If you are sending rifleman infantrymen to Los Angeles, you are sending a very definitive message about your intent. And I don't think that's a particularly peaceful message to send," General Parker told the ABC.
"I have seen a number of marine veterans expressing a great deal of concern here."
Are Trump's moves legal?
Marines have not been mobilised within the US like they are in California now since the 1992 riots in Los Angeles.
They were used for policing under the Insurrection Act after the California governor at the time asked President George HW Bush to help respond to riots over the acquittal of police officers who beat Black motorist Rodney King.
More than 50 people were killed in the riots, which also caused some $US1 billion in damage over six days.
This week, California sued the Trump administration to block deployment of the National Guard and the marines on Monday, arguing that it violates federal law and state sovereignty.
The president has more direct authority over the marines than the National Guard, under Title 10 and in his constitutional role as commander in chief of the armed forces, legal experts said.
A provision of Title 10 allows the president to deploy National Guard units into federal service if the US is invaded, there is a "rebellion or danger of rebellion", or the president is "unable with the regular forces to execute the laws of the United States".
General Barthel said we could also see this play out in the courts, given Mr Trump had gone behind the governor's back to bring in the National Guard and the marines were sent when the insurrection act had not been enacted.
"The courts may come back and say he overstepped his executive privilege," he said.
"This is the first time a lot of us are seeing what's going on in a situation like this. And so we'll see how it plays out."
