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.

The three-day bench trial before U.S. District Judge Charles Breyer, the brother of retired Supreme Court Justice Stephen Breyer, will determine whether the Trump administration violated a 19th-century rule prohibiting the military from civil law enforcement. It will be the first time the post-Reconstruction Posse Comitatus Act of 1878 will be tested in a courtroom.

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“The factual question which the court must address is whether the military was used to enforce domestic law, and if so, whether there continues to be a threat that it could be done again,” Breyer said at the start of the hearing.

The trial is an extension of a June lawsuit the state filed against President Donald Trump after he federalized California’s National Guard. At the time, Los Angeles Mayor Karen Bass (D) compared the mobilization to an “armed occupation.” Gov. Gavin Newsom (D-CA) and Attorney General Rob Bonta (D-CA) called it political theater and likened Trump’s actions to a dictator. They said sending in the National Guard only inflamed tensions. Trump, in turn, endorsed the idea of arresting Newsom.

Breyer oversaw the June hearing and initially sided with Newsom, ordering Trump to return power to the governor. However, a three-judge panel on the 9th Circuit Court of Appeals blocked the move, claiming Trump was within his right to call up troops to protect federal buildings and workers from protesters. The appellate panel did not weigh in on whether Trump’s order violated the Posse Comitatus Act.

The lawsuit has turned into a constitutional clash between Trump and Newsom, an early 2028 Democratic White House hopeful. How it shakes out could profoundly affect how the Trump administration uses military power to enforce its immigration agenda.

Maj. Gen. Scott Sherman, who oversaw the military involvement in Los Angeles, testified Monday that troops who were called in to assist with immigration raids were allowed to take law enforcement actions, such as setting up a security perimeter outside of federal facilities if necessary.

California lawyers also asked Sherman about his primary responsibilities. Sherman said they included ensuring troops’ compliance with the law, including the Posse Comitatus Act, ABC 7 reported.

Questioning then turned to communication between Sherman and the Department of Defense:

“Did you ever hear there was a rebellion?” a state lawyer asked. Sherman replied, “Not a rebellion, but that there were people preventing federal officials and law enforcement from doing their jobs and threatening federal buildings.” The lawyer pressed, “But no one from the Department of Defense ever used the term rebellion, correct?” Sherman responded, “Yes, correct.”

The Defense Department ordered about 4,000 California National Guard troops and 700 Marines. About 250 National Guard members remain.

William Harrington, the deputy chief of staff for the Army task force with tactical control over the Guard troops until last week, took the stand on Monday. He testified that those still on duty in Los Angeles were “supporting the request for assistance” from federal law enforcement agents. However, officials at the California National Guard said earlier in the day that the remaining troops mainly were on standby.

Since arriving in Los Angeles, federal agents have been sent all over the state to pick up illegal immigrants from bus stops, farms, Home Depots, and car washes.

California’s lawyers are expected to argue over the next two days that Los Angeles residents were “subjected to a form of military occupation” after Trump sent in the troops and that they worked in lockstep with federal immigration agents, “often indistinguishable from each other.”

“Never before, in the history of the Nation, has the federal government utilized the military for domestic law enforcement in this manner,” they argued in court documents ahead of the trial.

Specifically, lawyers in the state’s attorney general’s office pointed to more than half a dozen examples in which they alleged troops went too far and violated the Posse Comitatus Act.

California attorneys deposed an ICE field director who said National Guard soldiers went with immigration officers on about 75% of their missions.

In another instance, troops that went with a federal agency conducting a raid at a cannabis farm in Riverside County formed a security perimeter that prevented people from leaving. There were two other incidents in Los Angeles and Santa Barbara County where military service members detained civilians. Lawyers for the state argued that directly providing “security functions” for civilian law enforcement agents violates Posse Comitatus. They said that the military unit in Los Angeles was ordered to “actively provide security during civil law enforcement operations on a near-continuous basis since the deployment began.”

Lawyers for the federal government disputed the claims and the foundation of the case itself, arguing that Posse Comitatus is a criminal statute and can’t be used in the state’s civil suit against the Trump administration. The lawyers also believe neither Trump nor the Department of Defense overstepped their authority because the law does not prohibit Trump from using troops to protect federal property and personnel.

The Department of Defense issued a new activation order last week to keep troops in California for another three months. The order came two weeks after the Marines sent to Los Angeles left and a week after the Trump administration demobilized most of the National Guard.

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The San Francisco trial is taking place as Trump announced plans on Monday to send 800 National Guard troops to the nation’s capital to combat crime and said that the approach might be expanded to Oakland, New York, Baltimore, and Chicago, four cities run by Democrats. Though crime rates in Washington, D.C. have been falling, Trump claimed they are “totally out of control” and threatened a federal takeover.

The president did not mention the Republican-led states that are also struggling with crime.

The DC Guard is the only unit in the country that reports exclusively to the president.