Often it operates under state command and control, using state funding. Sometimes National Guard troops will be assigned by their state to serve federal missions, remaining under state command but using federal funding.

Its troops have been deployed for a variety of emergencies, including the COVID pandemic, hurricanes and other natural disasters. But generally, those deployments are carried out with the agreements of the governors of the responding states.

In the case of the Los Angeles protests, US President Donald Trump said he’d deploy 2000 National Guard troops over the objections of California Governor Gavin Newsom.

Here is what you need to know about the National Guard:

When was the National Guard established?

The National Guard dates back to the formation of citizen militias in the American colonies during the 17th century.

The passing of the 1903 Militia Act transformed into a better organised reserve force of the US military.

The National Guard was comprised of about 420,000 civilian soldiers in 2022, according to the US think tank the Council on Foreign Relations.

When can it be deployed by a US president?

Generally, federal military forces are not allowed to carry out civilian law enforcement duties against US citizens except in times of emergency.

An 18th-century wartime law called the Insurrection Act is the main legal mechanism that a president can use to activate the military or National Guard during times of rebellion or unrest. But Trump didn’t invoke the Insurrection Act last Saturday.

Instead, he relied on a similar federal law that allows the president to federalise National Guard troops under certain circumstances. He federalised part of California’s National Guard under what is known as Title 10 authority, which places him, not the governor, atop the chain of command, according to Newsom’s office.

The law cited by Trump’s proclamation places National Guard troops under federal command. The law says that can be done under three circumstances: When the US is invaded or in danger of invasion; when there is a rebellion or danger of rebellion against the authority of the US government, or when the President is unable to “execute the laws of the United States,” with regular forces.

The move appeared to be the first time in decades that a state’s national guard was activated without a request from its governor, a significant escalation against those who have sought to hinder the administration’s mass deportation efforts.

But the law also says that orders for those purposes “shall be issued through the governors of the States.” It’s not immediately clear if the president can activate National Guard troops without the order of that state’s governor.

When have troops been used before in the US?

The Insurrection Act and related laws were used during the Civil Rights era to protect activists and students desegregating schools. President Dwight Eisenhower sent the 101st Airborne unit to Little Rock, Arkansas, to protect black students integrating Central High School after that state’s governor activated the National Guard to keep the students out.

President George H.W. Bush used the Insurrection Act to respond to riots in Los Angeles in 1992 after the acquittal of white police officers who were videotaped beating black motorist Rodney King.