POD in the 60's
There is a way to do it.
A good POD would be in the 60's. Martin Luther King, Jr. is assassinated a few years earlier, let us say around 1965. As a result more militant factions develop in the African/American community. These factions starting with King's death use it to turn the cities of the U.S. into battle zones.
President Johnson is forced to draw down U.S. involvement in Vietnam because the troops are needed in our cities to maintain order. But this approach further angers many radicals. Joined by the Weather Underground and other radical groups a guerilla war begins in the U.S. urban centers. Enclaves of scared people, convinced that the government is unable or unwilling to protect them and convinced they must defend themselves begin to form "neighborhood protection groups," which are actually armed vigilantes patroling their neighborhoods. These vigilantes in some cities would get the nod from police due to strained police forces not being able to be everywhere at once.
The street warfare continues sporadically through the 70's and peters out by the 80's, but these urban militas remain "on guard," especially in the face of gang and drug related activity.
Geon