When Police View Citizens As Enemies:

The police and the U.S. military are separate institutions because policing a community and fighting a war are supposed to be separate jobs. In traditional “wars,” both sides are heavily armed. In Minnesota, only the agents of the state appear to be wearing body armor and carrying long guns. And yet: State officials are calling this “warfare” on official public channels.

I think it began with the “War On Drugs” in the 70s and 80s that turned the United States into an incarceration state.