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.

"Why is the federalized National Guard, even though it's been drawn down, still in place?" a federal judge asked.

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.