"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."