Political Era was from 1840 - 1920. It was when the police agencies we're first established to provide a unified law enforcement force in the major American cities.
Professional Era was from 1920-1970. There was a call for the establishment of measures to assist law enforcement agencies to improve their effectiveness and become more professional.
Community Era is from 1970 - the present. This era is advocated for law enforcement to move away from the crime fighting focus for law enforcement towards a greater emphasis on maintaining order and providing service to the community.