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.