SAN FRANCISCO — A federal judge has determined that former President Donald Trump crossed legal boundaries when he dispatched National Guard troops and Marines to Los Angeles last June, marking a significant ruling under an 1878 law aimed at limiting military involvement in civilian affairs.

In a 52-page decision issued Tuesday, U.S. District Court Judge Charles Breyer found that Trump’s mobilization of approximately 4,000 National Guard members and 700 Marines was not only unlawful but violated the Posse Comitatus Act, which prohibits military forces from enforcing domestic laws without congressional approval. The ruling followed a trial during which California officials argued that Trump overstepped his authority in responding to protests against his administration’s immigration policies.

Judge Breyer highlighted the characteristics of the military presence, noting that armed soldiers and military vehicles were deployed to establish barricades and exercise crowd control, actions that were deemed inappropriate under federal law. “There was no rebellion,” Breyer stated, emphasizing that local law enforcement was capable of managing the protests.

California’s request aimed to restore control of the troops to Governor Gavin Newsom and prevent further military engagement in enforcing federal laws within the state. While Breyer’s injunction pertains specifically to California, it underscores broader concerns about the potential militarization of law enforcement across the nation while also producing uncertainty regarding similar military deployments in other states.

The ruling allows the 300 National Guard troops currently stationed in Los Angeles to continue their mission of protecting federal properties under the Posse Comitatus Act, albeit under strict limitations regarding their roles. Breyer’s order prohibits the Trump administration from employing military personnel for a range of law enforcement activities unless authorized under exceptional circumstances.

Although the ruling is limited in geographic scope, Breyer expressed concern that Trump’s actions could set a precedent for creating a national police force centered around presidential authority. This could raise alarm about the balance of power between federal and state governance, especially when interacting with the National Guard in urban areas.

Following the decision, legal experts speculate that the Trump administration may seek to appeal, potentially leading to reviews by the U.S. Court of Appeals for the Ninth Circuit and even the Supreme Court. This legal battle could have lasting implications not only for California but for the entire nation as questions arise regarding the use of military resources in domestic situations.

The ruling arrives on the heels of heightened scrutiny surrounding military deployment in civil matters, challenging a norm derived from historical tensions between military and civilian governance. As discussions about public safety and policing continue, the outcome of this case could impact how future administrations approach military involvement in local law enforcement.