A federal judge in San Francisco seemed unconvinced after a three-day trial that the continued deployment of federalized members of California’s National Guard — who were originally deployed to Los Angeles in response to protests against President Donald Trump’s immigration agenda — is lawful.

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.