There’s a reason it’s hard to discipline police. It starts with a bill of rights 47 years ago.

On a Thursday afternoon in March 1973, 50 uniformed officers filed into a red-brick legislative building in the Maryland state capital, armed with stories of being wrongfully disciplined by highhanded police chiefs, gripes of low morale, and threats for lawmakers who didn’t agree to help them.

At stake was the “Law Enforcement Officers Bill of Rights” — a first-in-the-nation law that codified workplace protections for police officers far beyond those afforded to other government employees. They included giving officers a formal waiting period before they had to cooperate with internal inquiries into police conduct, scrubbing records of complaints brought against officers after a certain period, and ensuring that only fellow officers — not civilians — could investigate them.