A federal judge has declared President Donald Trump’s use of military troops in Los Angeles illegal, barring the Pentagon from using National Guard members and Marines from performing police functions, like arrests and crowd control.

In a 52-page ruling, U.S. District Judge Charles Breyer warned that Trump appears intent on “creating a national police force with the President as its chief.”

Trump billed his deployment of troops to Los Angeles, starting in early June, as a way of bolstering immigration enforcement efforts amid protests in the city against the president’s deportation agenda. Though Trump has now withdrawn all but 300 of those troops, he is mulling sending troops to other major cities, such as Chicago. He has also deployed the National Guard in Washington, D.C., under a separate legal authority from the one he used in Los Angeles.

Breyer, a Clinton appointee based in San Francisco, concluded that Trump’s L.A. deployment — an operation overseen by Defense Secretary Pete Hegseth — violated a longstanding law meant to prevent domestic law enforcement by the military: the Posse Comitatus Act. His decision followed a four-day trial last month that included testimony from the Pentagon officials overseeing the troop deployment in Los Angeles.

The Posse Comitatus Act of 1878 bars the military from enforcing domestic laws without explicit permission from Congress. But Breyer said that despite this restriction, the Pentagon “systematically used armed soldiers” to perform police functions. And he noted that Trump has expressed his desire to use similar tactics in other cities across the country.

Breyer’s ruling bars the Pentagon from “ordering, instructing, training, or using the National Guard currently deployed in California, and any military troops heretofore deployed in California” from “engaging in arrests, apprehensions, searches, seizures, security patrols, traffic control, crowd control, riot control, evidence collection, interrogation, or acting as informants,” without demonstrating they have permission from Congress.

Breyer put his ruling on hold until Sept. 12 to allow the Trump administration to appeal.