Armed force of a country intended to keep order among civilians. Usually the only formation allowed to use force to maintain the law. They usually wear uniforms, but some of police officers act as secret agents.
Questions related to history of police forces in general or in specific countries or territories should be tagged with the police tag. Also methods of operation of police (like criminology) should be marked this way.
For Military police (MP), internal force of army, intended to keep order between soldiers, add the military tag.
Questions related to other police-like forces, like the FBI, can also use this tag, if necessary.
For counter-intelligence forces the espionage tag could be more appropriate.