You've heard of the Weather Underground before, but it's most likely because Barack Obama's relationship with former WUO leader Bill Ayers became a huge controversy during the 2008 presidential election. Have you ever looked into what kind of shit that organization got up to when it was active, though? If there's any really simple way to grasp the enormity of the domestic terror problem in the 1970s, giving this list of confirmed WUO actions even so much as a quick glance should do the trick. Or just check out this screenshot:
I don't expect you to read this.
See how jam-packed it is? Well, that just covers the year of 1970. Granted, they were responsible for very few deaths (but definitely some), preferring to plant and detonate their explosive devices in those early morning hours when government employees aren't at work. I suppose that makes their activity a little less important in a historical sense, but still, can you imagine if one group pulled off a fraction of this stuff in the span of one year within the United States now? It would be absolute bedlam. We'd probably be living under martial law if that happened today.
Weirdly enough, the Weather Underground incident that claimed the most lives only killed the people who were planning it, but they just narrowly avoided taking out a way more high-profile target.