From The New Jim Crow, how and when US policing moved beyond racial terrorism to fully militarized policing:

The transformation from "community policing" to "military policing," began in 1981, when President Reagan persuaded Congress to pass the Military Cooperation with Law Enforcement Act, which encouraged the military to give local, state, and federal police access to military bases, intelligence, research, weaponry, and other equipment for drug interdiction. That legislation carved a huge exception to the Posse Comitatus Act, the Civil War–era law prohibiting the use of the military for civilian policing. It was followed by Reagan's National Security Decision Directive, which declared drugs a threat to U.S. national security, and provided for yet more cooperation between local, state, and federal law enforcement.