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 high-stakes trial began Monday in a California courtroom over whether the Trump administration violated federal law when it deployed thousands of National Guard troops and United States Marines to Los Angeles following protests over immigration raids in June.

Trump cited Chicago, Los Angeles, New York, Baltimore and Oakland, Calif. as “other cities also that are bad, very bad.”