That’s Posse Comitatus. The Posse Comitatus Act, passed by Congress and signed into law in 1878, removed the U.S. military from regular civil law enforcement, according to a 1987 history published by The Journal of Criminal Justice. A posse comitatus is a group of people who are mobilized by a sheriff to take action against lawlessness. Soldiers serving in the National Guard are “rarely” covered by the Posse Comitatus Act because they generally report to their state or territorial governor. That means they’re bound by the Posse Comitatus Act until they’re returned to state control, according to the Brennan Center’s analysis.