The US has a long history of political violence, including left wing violence. In the 60's-70's there were widespread terrorist attacks, in fact; there was, often times, essentially low level war in the streets. One example is the Weather Underground, but other groups were mailing explosives, kidnapping and ransoming people, just all sorts of crazy shit. It's really just over these last 30-40 years that this violence died down. People today seem to forget this.
https://en.wikipedia.org/wiki/Weather_Underground
