US President Donald Trump is pushing the bounds of military activity on American soil, but a 150-year-old law could spell trouble for his plans.
On Monday, Mr Trump said he was deploying the National Guard and taking over Washington, DC's police department to reduce crime in the nation's capital.
As hundreds of National Guard troops filed into Washington, DC on Tuesday, a judge thousands of miles away in California was hearing arguments about whether such an act violated federal law.
The three-day, no-jury trial is considering whether the Trump administration violated the 1878 Posse Comitatus Act when it mobilised the National Guard during protests against immigration raids in Los Angeles in June.
Experts say in both the California and Washington cases, there are clear limitations to the law's enforcement.
Here's what to know.
What happened in LA?
Protests erupted in Los Angeles on June 7 when Immigration and Customs Enforcement (ICE) officers arrested people at multiple locations, including workplaces, hardware stores and bus stops.
Despite objections from California Governor Gavin Newsom and city leaders, the Trump administration federalised the California National Guard and sent members into the city.
The Department of Defense ordered the deployment of about 4,000 National Guard troops and 700 Marines to Los Angeles.
The guard members accompanied federal immigration officers on raids throughout Los Angeles and at two marijuana farm sites in Ventura County.
Marines stood guard around a federal building in downtown Los Angeles, which includes a detention centre that was holding some of those arrested.
Most of the troops have since left, but 250 National Guard members remain on duty, according to the latest figures provided by the Pentagon.
California is asking the court to order the Trump administration to return control of the remaining National Guard troops to the state.
What is the National Guard?
It's a section of the military that can be used as reserves for the army and air force.
Each state has its own unit, which answers to the relevant state governor and the president.
Its members have been used to assist with crowd control in the past, but presidents have rarely deployed them without the relevant governor's approval.
Before Mr Trump in June, President Lyndon B Johnson was the last to do so in 1965.
What is the Trump administration accused of?
Judge Charles Breyer says the central question is whether the Trump administration may have violated the 1878 Posse Comitatus Act.
That federal law generally prevents the military from participating in domestic law enforcement.
It also prevents the military from investigating local crimes, overriding local law enforcement or compelling certain behaviour.
The law typically doesn't apply to the National Guard because members report to their relevant governor rather than the federal government.
But because the Trump administration took control of the Guard members, the Posse Comitatus Act came into play, according to the Brennan Center for Justice.
The state of California says the federal government violated Posse Comitatus when it deployed National Guard soldiers and US Marines to conduct law enforcement actions normally conducted by police.
At the time of the ICE protests, Mr Trump pointed to a provision called Title 10 that allows the president to call the National Guard into federal service when the country "is invaded", or when "there is a rebellion or danger of rebellion against the authority of the Government".
But Judge Breyer found the protests fell "far short of 'rebellion'".
He also previously found the Trump administration had violated the US Constitution's 10th Amendment, which defines power between federal and state governments.
The Trump administration immediately appealed, arguing that courts can't second-guess the president's decisions.
It secured a temporary halt, allowing control of the California National Guard to stay in federal hands as the lawsuit unfolds.
What has the court heard?
During the trial's first day, a handful of witnesses were called for testimony.
Deputy Commanding General for the National Guard Major General Scott Sherman said the military can protect federal property and federal agents in their mission carrying out federal operations.
He said they could take certain law enforcement actions, such as setting up a security perimeter outside of federal facilities, if a commander on the ground felt unsafe.
Ernesto Santacruz Jr, the Los Angeles field office director for ICE, argued in court documents the troops were needed because local law enforcement was slow to respond when a crowd gathered outside the federal building to protest the June 7 immigration arrests.
"The presence of the National Guard and Marines has played an essential role in protecting federal property and personnel from the violent mobs," he said.
Who would enforce the Posse Comitatus Act?
Beyond the legal exemptions to the Posse Comitatus Act, there is a question around how the law can actually be enforced, Brennan Center for Justice's Liberty and National Security Program counsel Joseph Nunn said.
Because the Posse Comitatus Act is a criminal statute, the US Department of Justice is responsible for prosecution in criminal court, Mr Nunn said.
"It's premised on the executive branch policing itself," he said.
That makes it unclear whether a state government such as California has a right to sue in a civil court in the first place.
Why does the trial matter?
Georgetown University Law Center professor of law Steve Vladeck said the ruling in the California case would likely be a narrow interpretation based on the circumstances of the Guard's deployment in Los Angeles.
But he said a precedent could still dictate how the administration used the Guard in California and other states.
On Monday, Mr Trump announced he was deploying the National Guard across Washington, DC, and has suggested he may do the same in Chicago.
But a court ruling against the Trump administration's use of the National Guard in California could put a pin in those plans.
Why has the Guard been deployed in Washington, DC?
Mr Trump said he was deploying the National Guard and taking over Washington's police department to reduce crime.
The president has warned of a takeover since a former Department of Government Efficiency employee Edward Coristine, better known by his online alias Big Balls, was allegedly assaulted during an attempted carjacking last week.
Washington falls under federal control, meaning Congress has ultimate jurisdiction over the city.
The president is also already in charge of the National Guard in Washington and can legally deploy troops for 30 days without congressional approval, as well as take control of the city's police department for 30 days if he determines there is an emergency.
However, Washington Mayor Muriel Bowser says the crime rate in the nation's capital is already falling after a spike in 2023.
ABC/AP
