Posse Comitatus Act at center of Trump-Newsom National Guard dispute in LA

President Donald Trump and California Gov. Gavin Newsom are clashing over the deployment of the National Guard amid ongoing Los Angeles riots with different interpretations of the 1878 law that governs the use of the military to enforce the law domestically.

The Posse Comitatus Act was signed into law by President Rutherford B. Hayes amid concerns that the military had too much sway over civilian affairs during Reconstruction. It generally bars the use of the U.S. military for civilian law enforcement inside the country, but there are key exceptions. Trump deployed the National Guard early Sunday, but so far, those troops have not participated in any direct law enforcement.

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Trump sent in the National Guard after Immigration and Customs Enforcement agents were reportedly attacked on the streets of L.A. as they conducted raids to catch and deport illegal immigrants. Seeing that neither Newsom nor L.A. Mayor Karen Bass were moving aggressively enough to stop the attacks, Trump signed a presidential memorandum to deploy 2,000 National Guard troops to "address the lawlessness that has been allowed to fester," the White House said in a statement.

Demonstrators block the highway as law enforcement officers face them during a protest against federal immigration sweeps in Los Angeles on June 8, 2025. (REUTERS/Mike Blake)

Newsom objected immediately even as the riots spiraled.

"I have formally requested the Trump Administration rescind their unlawful deployment of troops in Los Angeles county and return them to my command," Newsom wrote on X on Sunday alongside his letter to President Trump. "We didn’t have a problem until Trump got involved. This is a serious breach of state sovereignty – inflaming tensions while pulling resources from where they’re actually needed."

In addition to the Guard, the Pentagon said Sunday night it stands ready to send in 500 active-duty Marines.

If the military is to take an active role in suppressing the riots, the 147-year-old law will take center stage. Exceptions to the Posse Comitatus Act that allow the military to enforce the law against U.S. citizens include congressional decree, if an insurrection has been declared under the Insurrection Act, or when state governments prove unable or unwilling to impose law and order.