LOS ANGELES — A federal judge in San Francisco barred soldiers from aiding immigration arrests and other civilian law enforcement across California, warning Tuesday of a growing “national police force with the president as its chief” in an impassioned order set to take effect Sept. 12.

The decision is a major win for California and could have wide-ranging consequences for President Donald Trump’s use of the military nationwide. But legal experts warn it could prove a Pyrrhic victory, won at such great cost it is tantamount to defeat. Many fear a reversal on appeal could hand the president near unlimited power to deploy troops on American soil.

“The stakes are huge,” said Eric J. Segall, a professor at Georgia State University College of Law. “If this District Court decision is reversed by either the 9th Circuit or the Supreme Court or both ... the Trump administration would go hog wild.”

In a 52-page decision filed early Tuesday morning, Senior District Judge Charles R. Breyer blocked the administration from “deploying, ordering, instructing, training, or using” California troops to engage in civilian law enforcement, as Trump has done throughout Southern California during sweeping immigration raids since June.

“Why is the National Guard still around?” Breyer demanded with evident irritation at trial last month.

“What is the threat today? What was the threat yesterday or two weeks ago that allowed it?” the judge said at the time. “I’m trying to see whether there are any limits, any limits to the use of a federal force.”

Tuesday’s ruling comes as hundreds of troops now patrol the United States capital, following an order by the president in mid-August deploying the National Guard to tamp down what he described as rampant crime in D.C. Thousands more could soon be deployed to other American cities, the administration has warned.

About 300 soldiers remain on the streets of Los Angeles, where thousands of National Guard troops and hundreds of Marines were deployed in early June to quell fierce protests over immigration raids.

Breyer’s order would strictly limit what those remaining forces can do. The Department of Justice indicated it would immediately appeal the decision, all but ensuring a stay until the 9th Circuit can rule on it later this month. Trump said Tuesday that the judge had “ruled against our country.”

Legal experts say the ensuing judicial dust-up will clarify precedent in a murky corner of the law. But some warn it could also unearth a road map for future deployments in cities across the U.S., should California go on to lose in a higher court.

“If Breyer sides with (Gov. Gavin) Newsom, and the 9th Circuit sides with Trump, we now have a playbook to use the National Guard and maybe the military around the country,” said Mark P. Nevitt, a law professor at Emory University and one of the country’s foremost experts on the law at the heart of the case.

“He’d have a ruling from the most liberal circuit in America giving the legal go-ahead for this deployment,” Nevitt said. “That would make bad law for the country.”

Trump has long threatened to send troops to police American cities, especially those run by his political foes.

As president, he repeatedly raised the specter of using the Insurrection Act, an 1807 law that grants the executive branch sweeping domestic military power. So many court-watchers were surprised when the Department of Justice instead lifted the authority to federalize California’s Guard troops from an obscure subsection of the U.S. code.

In his ruling, Breyer called the decision a “tacit admission” by the administration that facts on the ground didn’t support their narrative — one more easily probed under the Insurrection Act than 10 U.S.C. 12406, the statute the Justice Department invoked.

Legal experts say the move is emblematic of the administration’s second-term judicial strategy.

“He’s using these shadow statutory mechanisms to get where he wants to go without making the hard political decision of invoking the Insurrection Act,” Nevitt said. “His lawyers are scrubbing the U.S. Code looking for executive power.”

The three-day trial in August revealed stunning new details of some of the military’s most controversial actions in Southern California this summer, including their participation in a July raid of MacArthur Park that enraged residents and city officials.

On Aug. 12, Maj. Gen. Scott Marshall Sherman testified that Border Patrol agents had initially planned to target the park on Father’s Day — a decision the military overruled, saying the expected crowds made it too dangerous.

“It was going to be a very large amount of people in the park,” said Sherman, who led the federalized National Guard troops and Marines sent to L.A. in June. “I could not approve it because of the high risk.”

Sherman’s initial refusal to rubber stamp the military mission led Chief Gregory Bovino of the Department of Homeland Security to question his loyalty to the United States, a fact California’s attorneys fought hard to see included in the court record.

Breyer himself was incredulous at the implication.

“If ... a military officer who’s had 33 years of experience is told by civilian authorities that he is disloyal to the United States of America, why isn’t that relevant to whatever decisions he made?” Breyer said at trial.

State and local leaders have repeatedly called the military’s actions “political theater” and decried their use as a distraction from everything from the Epstein files to Trump’s tariffs.

But experts warn California’s legal arguments are also tenuous, and could set up a crushing defeat in the weeks to come.

It’s the second time Newsom has risked expanding the presidential power he sought to curtail when he filed suit against Trump over the troop deployment in June.

On June 9, Breyer ruled to strip the president of command of federalized troops, saying he’d overstepped his authority under an obscure subsection of the U.S. Code. The 9th Circuit quickly reversed that decision, finding the president had broad discretion over domestic deployments.

Now, the appellate court must weigh whether the same broad presidential discretion extends to violations of the Posse Comitatus Act, a 19th-century statute that forbids soldiers from enforcing civilian laws in all but the most extreme circumstances.

The Department of Justice contends that once the president invokes his near-total authority to deploy troops, almost anything soldiers might do to “protect” federal law enforcement is permitted under the act.

“Are you saying because the president says it, therefore it is?” Breyer said during the trial. “In other words, we’re going to see federal officers everywhere if the president determines there’s some threat to the safety of a federal agent.”

The federal government’s argument during the August trial veered at times into what Breyer called “Alice in Wonderland” logic: Justice Department lawyers said both that Los Angeles troops had stringently followed the law and that the law did not apply to them.

“Why did I spend a day looking at slide after slide and regulation after regulation and report after report on ... compliance with the Posse Comitatus Act if the Posse Comitatus Act is irrelevant?” Breyer snapped at the Justice Department legal team. “Maybe you should tell your client that they don’t have to follow (it) if that’s your view.”

Likewise, administration attorneys told the court Trump can’t be sued for violating the criminal statute. But neither can he be prosecuted for breaking it, they said, thanks to the Supreme Court’s presidential immunity decision last year.

“So there’s no remedy,” Breyer said.

Reaction to Breyer’s Tuesday decision was mixed. Some legal experts say the law is unclear.

“The legality of all of this is really messy,” Nevitt said. “Arguably California might not have standing to even get to the merits of the case.”

Others were more bullish on California’s chances.

“This is an opportunity to give more meaning to a statute that’s notoriously vague,” said Dan Maurer, a law professor at Ohio Northern University. “It’s important to see what can the president get away with.”

State and local leaders celebrated Tuesday’s decision.

“Today, the federal court ruled in favor of the people of Los Angeles,” said Los Angeles Mayor Karen Bass. “The White House tried to invade the second-largest city in the country. That was illegal. Los Angeles will not buckle and we will not break. We will not be divided and we will not be defeated.”

But some legal scholars were less sanguine. Some fear expanding the use of soldiers for civilian policing could be a first step toward martial law.

“The reason Trump might find that delightful is because that’s what Lincoln did,” Segall said. “Trump wants to be Lincoln.”

The president has already signaled his intention to expand the use of the military.

“We’re going to look at New York. And if we need to, we’re going to do the same thing in Chicago,” Trump said during a news conference in August. “Hopefully, L.A. is watching.”

On Tuesday, he reiterated the threat, adding that he could soon deploy more troops to Los Angeles.

“(Newsom)’s going to need us again, because it’s starting to form again,” he told the White House press corps Tuesday afternoon. “I see it.”

For Breyer, the threat is existential.

“What’s to prevent a national police force?” the judge said. “Is there any limit?”

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(Los Angeles Times staff writer Michael Wilner contributed to this report.)

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