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Law enforcement is the job of the police, in most civilized countries. They are expected to investigate crimes, find the perpetrators, and present their case in a court of law, usually (in Common law countries) before a jury of the perpetrator's peers. Well, not exactly the perp's peers, but a general selection of normal people.
While this is a noble quest, it all too often breaks down.
Authoritarian regimes often employ various levels of "secret police" in order to patrol and regulate their populations. Such schemes often involve extra-legal violence or threats of violence, encouragement of people willing to turn in their acquaintances or enemies on flimsy grounds of wrongdoing, and less-than-transparent judicial processes.
Even modern democracies struggle with the tendency of law enforcement to overstep their remit. In a democracy, law-enforcement personnel are supposed to be operationally independent, so that in particular they can investigate anyone - up to and including the political leadership of the country - whom they suspect of having committed a crime, in any legal manner they feel appropriate, without fear or favour. Although in many countries sitting politicians have some degree of immunity from prosecution, it may still be possible to arrest them after they leave office. Sometimes, this actually works and corrupt presidents do in fact end up in jail.
Lack of operational independence can lead to law enforcement being used as a weapon to target people the government doesn't like. However, laws which - intentionally or unintentionally - have a biased impact on certain groups of people can also have a similar effect - as can institutional racism or corrupt leadership in a police force.
The first police department in the modern sense of the term was founded in France in the seventeenth century during the reign of King Louis XIV. It was the Paris police department, and was registered in 1667 by the French Parliament.