United States Police Forces
United States police were patterned largely after British police. The offices of sheriff and constable, which originated in England under Norman rule, were in existence in colonial America. London's police force, established in 1829 through the efforts of Sir Robert Peel, was the forerunner of modern urban forces. New York City established a professional police force in 1844; other growing cities soon established similar forces to replace elected watchmen. In many frontier communities of the 1800's, volunteers called vigilantes were the main police. State police were patterned after the Texas Rangers, organized in 1835.
Separate police agencies exist at the local, state, and federal levels. Each police agency sets its own standards for the selection, training, and performance of police. Most police agencies are small municipal units; many consist of only one, two, or three part-time police officers. Most of the country's police officers, however, are employed by large municipal forces.
State and federal troops form a reserve police force that can be called upon in emergencies. In case of riots, strikes, or disasters, state governors can call out National Guard units to aid or substitute for civil police. The regular armed forces, including federalized National Guard units, can be called out by the President to control civil disorders (at the request of a state) or enforce federal laws. But even when military units are used as police, an area is very rarely put under martial law (rule by military rather than civil authorities).
The federal government provides information, aid, and training to state and local police agencies through the Federal Bureau of Investigation (FBI) in the Department of Justice. The department also grants funds to state and local police agencies for research, planning, and demonstration projects.