The civil force a state authorizes to keep public order, prevent and investigate crime, and enforce the law through patrol, arrest, and the threat of force. The police are the state's hand on the street — the everyday monopoly on legitimate force, trusted to protect and feared for the same power. Modern policing has two genealogies: the London 'preventive' model of policing by consent, and, in the American South, the slave patrols that hunted escaped enslaved people. The institution is at once a genuine public good and a standing instrument of coercion, and societies argue endlessly over how to control what they have built.
Call out for help and the police come. Most people, most of the time, are glad they do. The same badge that answers the burglary can also kick in the wrong door, beat the wrong man, and arrest the protester for protesting. That is the bargain at the heart of the institution: a society hands a uniformed body the right to use force on its behalf, then spends the rest of its life trying to control what it has created. The modern police were born twice. In London in 1829, Robert Peel built a force that promised to prevent crime and to police by consent. In the American South, an older root ran through the slave patrol, the armed bands that hunted the escaped and enforced a racial order. The protector and the patroller share a lineage. The history of the police is the history of trying to keep the first from becoming the second.