For nearly 150 years, a core tenet of the U.S. military — enshrined in the Posse Comitatus Act — has been that American troops should not be used in domestic law enforcement, except in the most extreme circumstances and only when expressly authorized by Congress or the Constitution.

The 1878 law bars military personnel from making arrests, conducting searches, or seizing property. But most of the troops President Donald Trump has federalized have been limited to protecting federal property, including buildings and vehicles, and ensuring the safety of civilian law enforcement, in particular, personnel from Immigration and Customs Enforcement.