If President Trump deploys troops to other American cities, experts say it would be a clear departure from founding principles and invite a legal challenge.

A federal judge is hearing arguments on whether military troops deployed this summer by the Trump administration to Los Angeles violated a federal law that bars troops from conducting law enforcement duties within the country.

Lawyers for President Donald Trump and California Gov. Gavin Newsom are set to face off Monday to determine whether the president violated a 147-year-old law when he deployed the National Guard to quell anti-immigration enforcement protests in Los Angeles – against the wishes of the governor.