What Is the Posse Comitatus Act in California?

The Posse Comitatus Act in California is the federal criminal law, at 18 U.S.C. § 1385, that bars the use of the U.S. military to enforce civilian laws unless the Constitution or Congress specifically allows it. California has no separate state version, but the federal act took center stage here in September 2025, when a federal judge ruled that the Trump administration violated it by deploying federalized National Guard troops and active-duty Marines around Los Angeles. A willful violation carries up to two years in federal prison.¹

What the Law Prohibits

The statute is a single sentence. It makes it a federal crime to willfully use the Army, Navy, Marine Corps, Air Force, or Space Force to execute domestic laws, unless the Constitution or an act of Congress expressly authorizes it.¹ “Execute the laws” is the phrase courts focus on. It covers activities like arrests, searches, seizures, traffic stops, crowd control, and evidence collection when performed by military personnel on civilians.

Penalties include a fine, up to two years in federal prison, or both. Criminal prosecutions are rare in practice. The statute reaches the official who “willfully uses” the military, not individual soldiers following orders, and the far more common remedy is a civil injunction ordering the deployment to stop. Evidence gathered through a violation can also be suppressed in later criminal cases, which can collapse prosecutions built on it.

How the National Guard Fits In

Whether the act applies to Guard troops in California turns on which authority they are operating under. Guard members serve both the state and the federal government, and they can be activated under three different statuses.

Under State Active Duty, the governor calls up Guard members using state authority and state funds. The Posse Comitatus Act does not apply, and troops can help with law enforcement if state law permits. This is the status used for wildfires, earthquakes, and other state emergencies.

Under Title 32, Guard members may be performing missions requested by the federal government and drawing federal pay, but they remain under the governor’s command. Because they answer to the state chain of command, the act still does not apply. This arrangement has been called a loophole, since federally funded troops can perform tasks that active-duty soldiers could not.

Under Title 10, the president federalizes the Guard. Members become part of the federal armed forces, and the Posse Comitatus Act applies fully. They cannot participate in civilian law enforcement unless a specific exception authorizes it. That last status is what triggered the 2025 California litigation.

The 2025 Los Angeles Ruling

In September 2025, U.S. District Judge Charles Breyer ruled in Newsom v. Trump that the Trump administration violated the Posse Comitatus Act by deploying federalized National Guard troops and active-duty Marines throughout the Los Angeles area for law enforcement. It was the first time a federal court issued an injunction to stop an ongoing violation of the act.²

The court found that a military unit designated Task Force 51 had systematically engaged in domestic law enforcement across hundreds of miles over several months. Troops set up traffic blockades in residential neighborhoods, conducted crowd control at MacArthur Park, participated in a cannabis farm operation in Carpinteria, and maintained an armed military presence throughout the region. Soldiers were openly instructed that they could engage in some law enforcement actions, and the court found the violations willful rather than accidental. Judge Breyer called it a “top-down, systemic effort” to use military troops to enforce federal drug and immigration laws.³

The permanent injunction bars the administration from using federalized Guard troops or military personnel in California for arrests, searches, seizures, security patrols, traffic control, crowd control, riot control, evidence collection, interrogation, or acting as informants, unless a valid constitutional or statutory exception is satisfied.³

Exceptions That Can Make a Deployment Lawful

The act is not absolute. Courts read the exceptions narrowly, and the government bears the burden of showing an exception actually applies.

The Insurrection Act

The Insurrection Act is the main exception. It lets the president deploy military forces domestically in three scenarios: when a state legislature or governor requests federal help because the state cannot control domestic violence on its own; when rebellion or obstruction makes it impossible to enforce federal law by ordinary means; or when domestic violence or a conspiracy prevents residents from exercising their constitutional rights and state authorities cannot or will not protect those rights.⁴

Invoking it requires the president to personally issue a proclamation ordering the insurgents to disperse. That step is meant to make the decision deliberate and publicly documented. The act was last invoked during the 1992 Los Angeles riots.

Other Congressional Authorizations

Congress has authorized narrower forms of military involvement. Military personnel can give indirect support to civilian agencies through equipment loans, training, and intelligence sharing, so long as troops do not directly participate in searches, seizures, or arrests. The military can assist the Secret Service in protecting government officials and major political candidates. And federal law permits some involvement in counter-drug and border-security operations, though the 2025 ruling found those authorizations had been exceeded.

What It Means on the Ground in California

For residents, the practical line drawn by the ruling is fairly clean. When the National Guard is operating under the governor’s authority during wildfires, earthquakes, or other state emergencies, the Posse Comitatus Act does not restrict those operations. When troops are federalized and sent to enforce federal laws on California streets, the act prohibits that activity unless a specific exception like the Insurrection Act has been properly invoked.

The 2025 injunction applies to all military troops deployed in the state and permanently bars the administration from repeating the prohibited activities in California.² If you encounter military personnel involved in arrests, searches, or other law enforcement in California, that activity is now presumptively unlawful absent a specific statutory or constitutional authorization, and evidence obtained through it may be suppressed in any criminal case that follows.

1
Office of the Law Revision Counsel. 18 USC 1385 – Use of Army, Navy, Marine Corps, Air Force, and Space Force as Posse Comitatus
2
State of California – Department of Justice – Office of the Attorney General. Attorney General Bonta Secures Court Ruling Finding That Trump’s Use of Military Troops Violated Posse Comitatus Act
3
FindLaw. Newsom v Trump
4
U.S. Government Publishing Office. 32 CFR 215.4 – Responsibilities and Rules of Engagement