California is invoking a rarely used 19th-century law rooted in America’s post-Civil War history to challenge President Donald Trump’s deployment of troops in Los Angeles, arguing it violates federal limits on military involvement in civilian law enforcement.
At the heart of the legal battle is the Posse Comitatus Act of 1878, a statute passed in the aftermath of Reconstruction that prohibits federal troops from engaging in domestic policing. U.S. District Judge Charles R. Breyer ruled this week that California can move forward with a limited investigation into whether Trump’s use of the National Guard and Marines in Southern California breaches the act.
“President Trump’s continued use of federalized forces is directly relevant to the Posse Comitatus Act,” Breyer wrote Wednesday, authorizing expedited evidence collection.
The ruling comes after days of unrest in Los Angeles following ICE raids in Latino neighborhoods, which escalated into violent protests. Trump, accusing local leaders of losing control, bypassed California Governor Gavin Newsom to deploy the National Guard and Marines, a move that state officials say violates constitutional boundaries.
Breyer had earlier tried to limit Trump’s authority through a broader injunction, but that decision was overturned by the 9th Circuit Court of Appeals. This latest effort narrows the focus to whether Trump’s deployment crossed the legal line established by Posse Comitatus.
A Law Born from Bloodshed
The Posse Comitatus Act was passed during one of the most volatile chapters in U.S. history—following the Civil War and amid violent resistance to Reconstruction in the South. Its intent was to prevent the military from being used as a domestic police force, a role it had played during efforts to enforce the Fugitive Slave Act and protect Black voters during Reconstruction.
Historian Mark P. Nevitt calls the law’s origins “ignoble,” noting it was inserted into a spending bill by Southern Democrats in 1878 who had regained control of Congress and wanted to block federal intervention in local governance. The law effectively marked the end of Reconstruction and paved the way for Jim Crow laws by stripping federal troops from Black communities under threat.
While the law’s text is brief—just over 60 words—its implications have resonated for more than a century, with courts generally agreeing that using the military for civilian law enforcement is unconstitutional, except in cases of rebellion or emergency.
Trump’s Legal Justification Echoes Fugitive Slave Era
Legal scholars have drawn attention to the Trump administration’s use of obscure constitutional provisions to justify its actions—some tracing back to enforcement mechanisms used in the Fugitive Slave Act, a law that required the return of escaped slaves to their Southern owners and used federal force to do so.
“It’s a strange twist of history,” said historian Josh Dubbert. “Southern leaders were once eager for federal force to uphold slavery. Now Trump’s legal team is citing the same structures to round up immigrants.”
The Department of Justice argues that Posse Comitatus doesn’t apply in this case, citing legal loopholes—especially regarding the National Guard. When under state control, the Guard isn’t bound by the law, but when federalized, the rules become murkier.
Despite the historical weight behind the statute, legal experts are divided on how much ground California has to stand on.
Will the Case Hold?
Some civil liberties advocates believe the case is strong. “You have military units walking the streets of Los Angeles with civilian police,” said Shilpi Agarwal, legal director for the ACLU of Northern California. “That’s exactly what Posse Comitatus was meant to prohibit.”
But Nevitt is more skeptical. He notes that even if Breyer sides with California, the conservative-leaning 9th Circuit may overturn the decision—and if it reaches the Supreme Court, Trump is likely to prevail.
“The court could interpret the Guard’s role narrowly or rely on exceptions that gut the law’s effectiveness,” Nevitt said. “It’s going to be an uphill battle.”
For now, California’s legal team is pressing forward, hoping to revive one of the oldest legal limits on military power in U.S. history—one originally meant to prevent tyranny, now being used to push back against what they see as presidential overreach.
