A federal judge’s ruling that President Trump has been using troops illegally to perform law enforcement functions in Los Angeles will — if it stands — pose impediments to any plans Mr. Trump may have for sending the military into the streets of other cities, like Chicago.

Mr. Trump has made those threats in the context of his anti-crime operation in Washington, D.C., which has involved both civilian federal agents and National Guard troops under federal control. But because the District of Columbia is not a state, the federal government has greater latitude to use the Guard there.

The Posse Comitatus Act, enacted in 1878, makes it illegal to use federal troops for domestic policing under normal circumstances. So to keep from running afoul of that law, Mr. Trump would now need an additional legal rationale for deploying troops to cities like Chicago.

One potential model for Mr. Trump might be the reasoning his administration offered for sending troops to Los Angeles over the summer, ostensibly to protect federal agents and facilities. But on Thursday, Judge Charles Breyer of the Federal District Court in San Francisco held that the administration has been using those troops too expansively.

The judge barred the federal government from using troops anywhere in California to engage in “arrests, apprehensions, searches, seizures, security patrols, traffic control, crowd control, riot control, evidence collection, interrogation, or acting as informants.” The order is scheduled to take effect Sept. 12.

There are reasons for caution at this stage. An appeals court has already overturned an earlier decision by Judge Breyer, in which he tried to strike down Mr. Trump’s assertion of federal control of California National Guard troops over the objections of the state’s governor, Gavin Newsom. The Trump administration is likely to ask a higher court to reverse the latest decision as well.