A Senate Committee Probes FBI Cointelpro against the New Left, 1976

[Between 1956 and 1971] "The FBI conducted a sophisticated vigilante operation aimed squarely at preventing the exercise of First Amendment rights of speech and association, on the theory that preventing the growth of dangerous groups and the propagation of dangerous ideas would protect the national security and deter violence...

Many of  the techniques would be intolerable in a democratic society even if all of the targets had been involved in violent activity, but COINTELPRO went far beyond that.  The unexpressed major premise of the programs was that a  law enforcement agency has the duty to do whatever is necessary to combat perceived threats to the existing social and political order.