What to know about the 150-year-old law limiting Trump’s use of troops in US cities

Federal agents stage at MacArthur Park, July 7, 2025, in Los Angeles. (AP Photo/Damian Dovarganes, File)

President Donald Trump said Tuesday that he’s ready to order federal authorities to mobilize and combat crime in Chicago and Baltimore, despite staunch opposition from elected leaders and many residents in both cities.

Trump has already sent National Guard troops into Los Angeles and Washington, where he’s also federalized the police force. He has said he plans similar moves in other Democrat-run cities even as a federal judge on Tuesday deemed the California deployment illegal.

The Posse Comitatus Act is a nearly 150-year-old federal law that limits the U.S. military’s role in enforcing domestic laws. At its core, experts say the law reflects America’s long-standing belief that law enforcement should remain in civilian hands, separate from military power.