https://naacp.org/find-resources/history-explained/origins-modern-day-policing
"Slave Patrols continued until the end of the Civil War and the passage of the 13th Amendment. Following the Civil War, during Reconstruction, slave patrols were replaced by militia-style groups who were empowered to control and deny access to equal rights to freed slaves. They relentlessly and systematically enforced Black Codes, strict local and state laws that regulated and restricted access to labor, wages, voting rights, and general freedoms for formerly enslaved people.
In 1868, ratification of the 14th Amendment of the U.S. Constitution technically granted equal protections to African Americans — essentially abolishing Black Codes. Jim Crow laws and state and local statutes that legalized racial segregation swiftly took their place.
By the 1900s, local municipalities began to establish police departments to enforce local laws in the East and Midwest, including Jim Crow laws. Local municipalities leaned on police to enforce and exert excessive brutality on African Americans who violated any Jim Crow law. Jim Crow Laws continued through the end of the 1960s."